The error detail that's coming up indicates something on your local C:\ drive that the machine is looking for, and not finding.

The error appears to be confined to a few people, and doesn't seem to be a "mudcat" error.

The "null" call in the filepath, c:\!webserver\mudcat\htdocs\null, suggests a common viral "trick" where screen displays are sent to "null" to prevent you from seeing that something's going on. (In which case it would normally appear as c:\!webserver\mudcat\htdocs>null, but this isn't the only "standard" use for a "null address.")

I'm not able to find a specific infection that I can link directly to the symptoms described; but in ALL CASES OF UNEXPECTED BEHAVIOUR a recommended step is a FULL ANITVIRUS SCAN with CURRENT AV SIGNATURES. (Shouting intentional).

If you don't have a current AV on your machine that has been updated within the past week, Norton Free Scan is one of several places where you can get your machine scanned.

Use the program/provider of your choice; but the behavior described - limited to a few users who get unexpected results - suggests that a scan is in order.

You have "unidentified" files that are not readily found. You didn't put them there. Suspicion is merited.

John