If you have a CD burner or extra partition/harddisk, copy AVG install to it.
Also get Firefox from
www.getfirefox.com and the free Sygate firewall from
http://soho.sygate.com. Copy them over as well.
Get XP SP1a (as M$ calls it now) from
http://www.microsoft.com/windowsxp/downloads/updates/sp1/default.mspx
Get the 134mb Network Installation and copy that over as well.
Or better SP2, which includes SP1.
http://www.microsoft.com/downloads/...BE-3B8E-4F30-8245-9E368D3CDB5A&displaylang=en
This one is about 260mb. Also freely available on CD from MS or borrow it from someone else. If download, copy also to CD/HD.
Then disconnect the PC from the internet, and reinstall XP, (using the format partition option).
Then install (in this order) the motherboard-drivers, graphics drivers, other drivers you need, SP1 or SP2, Firefox, AVG (do not yet update), Sygate.
Only now connect the cable for the internet again. Run AVG and do an update of virus-definitions. Do a full scan of the PC.
Go and get Adaware and Spybot S&D, and install them. Source and instructions can be found in my post here:
How to remove Begin2Search/Coolwebsearch and Other Nasties
Have fun.