The following are possibilities you may experience when you are infected with a virus. Remember that you also may be experiencing any of the following issues and not have a virus.

Once the hard drive is infected any disk that is non-write protected disk that is accessed can be infected.

Deleted files
Various messages in files or on programs.
Changes volume label.
Marks clusters as bad in the FAT.
Randomly overwrites sectors on the hard disk.
Replaces the MBR with own code.
Create more then one partitions.
Attempts to access the hard disk drive can result in error messages such as invalid drive specification.
Causes cross linked files.
Causes a "sector not found" error.
Cause the system to run slow.
Logical partitions created, partitions decrease in size.
A directory may be displayed as garbage.
Directory order may be modified so files such as COM files will start at the beginning of the directory.
Cause Hardware problems such as keyboard keys not working, printer issues, modem issues etc.
Disable ports such as LPT or COM ports
Caused keyboard keys to be remapped
Alter the system time / date
Cause system to hang or freeze randomly.
Cause activity on HDD or FDD randomly.
Increase file size.
Increase or decrease memory size.
Randomly change file or memory size.
Extended boot times
Increase disk access times
Cause computer to make strange noises, make music, clicking noises or beeps.
Display pictures
Different types of error message