Well, it sounds like your computer has been infected with the issass back door trojan. You can go into your system folder and remove the file from the command line but I can't guarantee that that would work. If you're having this kind of trouble the issas has already corrupted your registry. Without going through the registry and removing all the entries for the issas it'll just keep reinstalling itself. And since you're having boot troubles that means removing the drive and hooking it up to another computer externally. Even then you'd need the right utilities to bypass all the windows user permissions. It's a real pain in the butt.
Reformatting your computer MAY help but it may not. The problem with these trojan viruses is that they often embed themselves in the boot partition on the hard drive. Reformatting doesn't actually destroy the boot partition. So it may reinstall itself when you reinstall the operating system.
What I would suggest is using Darik's Boot and Nuke. This utility will overwrite the entire hard disk with zeros. It's the only way to truly destroy the data. Then reinstall XP.
Darik's Boot and Nuke (Hard Drive Disk Wipe)
Be warned... DBaN truly destroys the data on the disk so if there is anything on there that you want or need I would suggest taking the drive to a professional and have them back up any important data.
Hope this helps.
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