@Lukas:
If the master boot record of an hard-disc is destroyed, you can repair it using the Windows Installation Disc.
Just insert it, boot from CD, go to "Repair Windows" and run "fix mbr". At least if I remember correctly from doing so after having crushed mine once

@Puppeteer:
So you have multiple hard-drives or one hard-drive with multiple partitions and your anti-virus software destroyed "something" which resulted in you not being able to see the partition/hard-drive and/or access the partition/hard-drive?
To me it sounds as if just the partition-table was somehow deleted.
However I personally have no idea as how to recover it.
But maybe google does?