Assuming that you want to run something other than NT, you will need to delete the partition(s) since they are probably NTFS. You will need a boot diskette from DOS or Win9x that also has FDISK.EXE and FORMAT.COM on it. This can be created from another machine. Boot the NT PC from this floppy, run FDISK from the A: prompt, follow the menus to delete all "Non-DOS" partitions.
Reboot the machine from the floppy again, run FDISK and follow the menus to create a "Primary DOS" partition.
Reboot again and format the C: drive with the "/s" switch.
Alternatively, if you are loading Win98SE and have the CD and Boot Disk, once the partitions are deleted, you can just boot from either of these, it will go directly into setup, and setup will allocate and format the drive for you.
Complicated - hope this helps...

Bill
CNE, MCSE, CCNA, CCA, MCT, CCI
"I'd much rather be lucky than good"