I have Windows 98SE. If I try to open a .txt file, a virus pops up c:\windows\system\checking.exe This is very annoying. I downloaded ad-aware, but it cannot complete the scan - after about 30k files, processor grinds down to a halt, and task manager tells me adaware in "not resonding". I use other anti-virus software such as AVG, but doesn't seem to pick anything up. Any idea's?

I also have a virus that AVG doesn't pick up. Have you tried running the online scans?

One thing I will try as soon as convenient will be to install the trial version of ewido. Give it a shot and let us know how it works.

A couple of online scanners:
http://www.pandasoftware.com/activescan/
http://housecall.trendmicro.com/

In addition to the online scans, run your spyware/antivirus scans in Safe Mode.
How to start Windows in Safe Mode