Reformatting the partition or partitions won't, by itself, remove viruses from the MBR (mind you, do viruses still live in the MBR these days?).

In theory, reformatting partitions isn't necessary if you just wipe all the executable files (EXE, DLL, all the other types an anti-virus will still scan if you deselect "scan all"); that way, you could keep all your data files. But then, if you don't have good backups of your data files, viruses are the least of your worries. Though on the gripping hand, modern viruses that act like fungi by lying hidden for a while before fruiting, might exist on your backups too.

Does Windows have an equivalent of Unix's ability to mount a filesystem noexec? If so, it'd be a neat trick to install Windows and all the applications to C:, and put all the data on D: and mount it noexec. Then you could wipe C: and the MBR and be sure you'd eradicated the virus. Mind you, that wouldn't work if you caught a virus while logged-in as an administrator, as presumably it could just turn noexec off again on D:.

Peter