I download stuff to a 8 gig hard drive. Only after being scanned (And passed) does the file get transferred onto the other hard drives. After the transfer the 8 gig hard drive gets formatted and made ready for the next download. It is rather extreme but two years and running I havent gotten infected yet. (Knock on wood)
If I did, rebuilding the machine takes a few hours no problem.
Enough about me… here are my thoughts for your issue:
Get a anti virus program and UPDATE it.
Run a anti virus scan, run adaware. Run spywareblaster.
Clean your machine of any infections.
Clear your temp internet files, clear all of your cookies and clear your history.
Defrag your hard drives.
Select check hard drives for errors after restart.
Reboot. Wait for machine to finish checking your drives for errors.
Log onto Microsoft update and update. It may challenge your machine for a Validation which it will determine if your XP pro key is valid and not pirated. Then you will update your machine.
Download the latest version of the program you are after. Immediately after downloading, scan the folder you put the download into with a (An updated and current) antivirus, (And updated and current) adaware type program. If you turn up no infections only then you are able to run the program and install it.
Sometimes windows firewall on XP pro will block the program and present you with a dialog box asking what you want to do with this program? Unblock? Keep blocking or remind me later. Im behind a router with NATS and disallowing unsoliticated traffic so the windows firewall and the other software firewall has nothing to do most of the time.
Maintaince is a hassle and time consuming. I think I dedicate one day per week and two hours each day (While Im asleep) the machine does tasks to take care of itself.
If you DO have a infection… well… it will require research as to what kind of infection and which file had it. Then you might understand how it go