Startup

Here are some things to try:
(Type the Q number into the ?For solutions containing...? box at support.microsoft.com)

"Invalid System Disk" After Setup Reboots

Q128730

If it?s the AntiCMOS.A virus, here?s Norton?s description:
http://securityresponse.symantec.com/avcenter/venc/data/anticmos.a.html

(There are no spaces in the URL)

Invalid system disk, means Windows can not find a valid boot sector on any of the boot devices specified by the BIOS.
This can be caused because the BIOS has forgotten the HD settings due to a weak/bad battery.
This can be a virus. (Boot on the AntiVirus floppy, and scan.)
This can be a failing hard drive.
This can be a bad setting in the BIOS. (Is the BIOS set to boot from SCSI for example.)
There are many possibilities.
Try the above, and post a comment about what happened.
You obviously have some troubleshooting in front of you.
Regards, Chris