i see this alot, it is virus based. even if you remove the virus and repair the system never acts quite the same. you can format some and the problems may be different but there are still odd problems. the only sure-fire way to fix this i have had any luck with is secure erase/low level format/packet write, which ever name you use, they are all the same thing. it erases the hard drive back to raw state(like a new hard drive) the boot sector is erased as well. then format and install. you may have to try a few things to get it to take the format. if you have a floppy drive use a disk utility that has secure erase options to boot the pc and erase. if for some reason it wont allow the format use a windows 98 se boot disk, fdisk at the A:\ prompt and delete all the partitions then boot and run the secure erase. just deleting the partitions does not remove info on the drive, using windows format options does not either. a good packet write/secure erase will take hours to run(well worth the effort) then just boot with the os cd and follow the directions to format and install. you can get any bootdisk from
http://www.bootdisk.com/
i use the secure erase option from a maxtor diagnostic disk, but many are availible and there are many free ones on the net.
years ago this was common practice to secure erase the disk before a format and reinstall.