No, you need more than a simple reformat. Virii can install themselves to the boot sector of your HD, and a format won't touch that part of the disk.
Fdisk will let you remove (or create) the partitions that are on the HD. If you remove all partitions, this also removes the boot sector, leaving you with a (practically) virginal HD.
Read up on fdisk. Do not guess while using fdisk, make sure you know what you're doing. Fdisk will destroy all the data on your HD in the blink of an eye, with no going back.
To use fdisk:
1. Go here:
and download the Windows98SE OEM file. Once downloaded, doubleclick it to create a bootable floppy disk that has fdisk on it.
2. Boot the pc with the floppy disk you created in step 1.
3. Type
fdisk at the command prompt.
4. Remove all partitions on the disk.
5. Reboot into your Windows CD, let Windows repartition and format the drive, load Windows.
6. Install drivers and applications.
Going on vacation in 4 hours...