Stop 50

I posted this message to comp.os.ms-windows.misc, but no one answered.

I was working on someones computer (the standard virus/spyware job), and gave it back to them. As soon as they got it home, they got a stop 50. I took a look at it, and it said:
STOP 0x00000050 ([changing variables])

This occurs as soon as the user selection screen come up. I searched the web and Microsoft's website, and tried some of their ideas. It's not Haxdoor, Spyware.Service.MiscrosoftUpdate, or Roxio.

Note:
1. this started RIGHT after I finished my job.
2. It's not built-in hardware.
3. It's a dimension 2400, and this happens in another one.
4. Safe mode works

Desperately in need of help,

poly-p man
.