Once you complete installing windows, or reinstalling windows, the next step is to get necessary tools and applications. You could install them if you have got an applications and drivers cd/dvd or you can do it from internet. I am giving here a step by step guide to get your computer up and running.

I suppose that you have completed installation and also you have connected to internet.

Windows Update

You should be able to open internet explorer.
The first thing that you will require is windows update
Open Internet explorer

Click on Tools > Windows Update

The browser will connect to Microsoft Windows Update site. And it will download a few activex components, you will see a small bar below the internet explorer's toolbar, when you click on it, it will display option to download activex components, you will have to click on it in order to download and install the activex component. Once it is complete, then it will start searching for installed updates. Then it will display a list of available Updates. Select Express Install which is also the recommended method displayed on the screen. The time to complete the process depends on the speed of the internet connection. Wait till the Updates are complete.

Windows Defender

The next important software you need to download from Microsoft's website is Windows Defender.

Windows Defender is Microsoft's antispyware application, it has a realtime scanning feature and auto update. It is a recommended software. But you will have to verify on Micorsoft's website that your copy if Windows is Genuine before you can download the software.

Click on the link below to visit Windows Defender website

http://www.microsoft.com/athome/security/spyware/software/default.mspx

Here you will be asked to validate the windows on yout computer, if you click on Validate button, it will download an activex component and you have to allow it ti download it and install , again look for the yellow bar under your internet explorer's toolbar, click on it and then select to install activex component. It will check the computer for Genuine Windows and display a thank you message and the download link to Windows defender. Click on the download button, again an alert is displayed asking you to either run or save the file. If you have a good internet connection and you do not want to save the file on your hard disk, then you can select Run, which will save the file in a temporary folder and then run the installation . In case if you save the file on your hard disk, then you will have to doubleClick on the file to start installation after the download is complete.

During Installation , you will be asked to join the Microsoft's Spynet, either Basic Membership or Advanced Membership. You have also an option Not to join it. Basically it allows you to send information of detected and unidentified spyware to Microsoft so that they can find a cure for it.

After the installation is complete, Open the windows defender program from Programs Menu.

On the Windows Defender window, click on the Downward Arrow next to the Icon of Question Mark. This is (Help Options) you can see a tooltip if yo keep your mouse over the icon for a few seconds. Clicking on the arrow will open a menu , where the second menu option from the top is called “Check for Updates”

Click on it. That will begin the Windows Defender Update. After the Update is complete you should be able to see a message to that effect.

Firewall

The next important software that you might consider downloading is a Firewall. Windows XP has a built in firewall, but it is not verbose, and you do not know actually what is happening on your computer while you are using the internet. You can download a separate Firewall program for that. You have a choice of getting Comodo Firewall or Zonealarm.

You will require an antivirus software on your computer. There are several choices among freeware like AVG, AVAST, AVIRA etc. 

Avast Antivirus Home Edition.

http://www.avast.com/eng/avast_4_home.html

Install (you can either download it and then run installation or run installation from a temp folder without saving it) Do not forget to complete the registration process by entering a valid email address, where you will receive a code, which you can use to enter in the program. Right click on the Avast icon in the taskbar > Select “About Avast”

> Click on License Key > Copy and paste the license key that you received in your email > Press Ok > Press Ok

Now you will be able to use Avast for a full year. If you ever reinstall windows again and if you need to reinstall Avast again, you can retrieve your license key by sending a request to Avast on the Avast registration page.

Avira Antivir Personal Edition. Visit website

AVG Antivirus Home Edition. Visit website 

BitDefender 10 Free Edition . Visit website

ClamWin Free Antivirus
(This is a GPLed, Open source Free Anti Virus, the only drawback currently is that it does not come with real time scanner. A good backup antivirus for those who are not content in having only one antivirus program on their pc) Visit website

Antispyware: 

spybot s&d anti spyware
Safer Networking

avg free antispyware (now it is part of AVG antivirus)
Free Grisoft

ad aware se free Antispyware
Lavasoft Antispyware

Arovax Free AntiSpyware 2.0.65
Arovax Antispyware

Super Antispyware Free Edition
SUPERAntiSpyware Free

Temp files / Registry cleaner

The next free sofware that you will require on your computer is a temp files and registry cleaner.

CCleaner :Ccleaner server the purpose best. You can download Ccleaner from the link

http://www.filehippo.com/download_ccleaner/

nCleaner : This program has more features than ccleaner, try if you like. Visit website

Firefox

By this time you have used the browser to visit trusted sites, but as you wander in the cyberspace you will require more protection than internet explorer can provide. If you already haven't made up your mind, it is time to install firefox. You can install Firefox from the link below.

Http://www.firefox.org

There are several good add-ons to firefox , you can choose from several hundreds of good plugins for several purposes.

Office Suite

If you are not in possession of MS Office, then you can install Open Office free from this site

http://www.openoffice.org

These are the few freeware that can make your computer ready to use after reinstallation of windows.