>How can I unpartition drive D (without commercial software), format drive C and install XP again?

Depending on how/when you partioned your old XP it might be hard to get rid of those partitions with older install/fdisk software.

The surest way I know is to get the disk checking / error handling software from your hard disk manufacturer (AFAIK always free) which can also erase everything on the disk. Usually works from boot disks on DOS or bootable el-torito-CD's. Before I install something on an old disk, checking the drive fitness can save me some work - but I hate installs <g>. Nulling out 20GB doesn't take too long. After that you are always asked to re-partition <bg>.

HTH

thomas