How technically competent are you? If it's increasing in frequency, it's probably a virus; many find, disable, and replace virus scanners as one of their top priorities.
You might try a free online one, such as
This one (pretty much must be run from IE; sigh) as something you just picked up is less likely to be affected (but not impossible).
Are you up to:
1) shutting the thing down
2) opening the case
3) removing all hard disks
4) opening another machine
5) putting the potentially infected hard disks into the second machine as non-booting drives
6) having the second machine scan the disks of the first
7) then putting everything back the way it was
?
A much simpler option is to back up non-executable stuff you want to keep, wiping the machine, and re-installing everything, of course; but the above preserves all data (at the cost of a small risk to the second machine)
An even simpler option (for you, anyway) is to phyisically take the thing to an expert... but that tends to cost money....