3dmark is a benchmarking program. I believe it's a free download.
As for the difference between format and clean install. I think they are one in the same. You need to format your old installation, and then install winxp fresh. You can use the xp disc dell gave you to do it.
-Boot to the CD
-Go through the setup untill you see a screen that says what partitions you have. I assume you'll only have a C: partition.
-You can delete that partition by pressing...