I noticed today the appearance and disappearance of a white box, almost like the behavior of a DOS window, but it wasn't a DOS prompt. I've scanned with Avast! and haven't found any viruses. Is it possible that this was a Trojan? The only other possibilities I could think of besides that was that it was something I did and didn't notice or it was my imagination. Please tell me I won't have to reinstall the OS.
AMD Palamino core 1.2 Ghz
Windows XP Home w/SP1a
20 GB Quantum Fireball with 10 GB of space the partition for Windows, and 10 GB of unpartitioned space
256 MB RAM
Nvidia GeForce 2
Nothing was in CD/DVD or floppy drives at the time.