The following are some common symptoms if your computer is infected with a virus

1. Computer does not boot.
2. Computer hard drive space is dramatically reduced.
3. Applications will not load.
4. An application takes longer to load than necessary or longer than normal.
5. Hard drive activity increases, especially when nothing is being done on the computer.
6. An anti-virus software message appears.
7. The number of hard drive sectors marked as bad steadily increases.
8. Unusual graphics or message appear on the screen.
9. Files are missing or deleted.
10. A message appears that the hard drive cannot be detected or recognized.
11. Strange sounds come from the computer.

Src: Cheryl A. Schmidt. A+ The Complete Guide to PC Repair. El Granada: Scott/Jones Inc., 2004.