It sounds like you're concerned that someone will be able to retrieve some data off of your hard drive. I'm not a computer genius, but I would use the delete and erase option of partition magic to get rid of the stuff you dont want. Then back up the files that i wanted to keep. (pictures, music,...). Finally i would format the hard drive and reload windows with the installation CD.
To format the hard drive, you'll have to start your computer in MS DOS mode and at the prompt "C:/" type in "Format C"
I think it should look like this: C:/Format C
the C might have to be in quotations.
It may work the way you have it. I don't know. I dont have a lot of experiance with Partition magic or Win 98SE