Virus symptoms.

Some of the symptoms that tells your system is probably affected by Virus

Data corrupted, altered or become unreadable
Programs won’t start or executes slower.
Hard disk space is mysteriously disappearing.
New error messages pop up on screen.
The date and time stamp or the size of program files change automatically.
Computer becomes slower than before.
Numerous copies of same file have been automatically created.
Computer hangs.
Floppies get corrupted and gives message like, track 0 bad.
BIOS gives message like something tries to write to boot sector.

Beware of these activities because you may not know how powerful the attack is going to be.