1) make sure you have saved any data from the disk first.

2) as a complete newbie you are likely to not understand most of this, but I'll start off anyway, you can ask questions.

You need a Win98 CD & 'Boot Disk' floppy - if you have a legal copy, you will have these - if you don't have a legal copy, I'm not going to help you, as I don't condone theft.

Put the boot floppy in the machine and sturn teh machine on, or do a reboot - push the button on the case that does that, or hit Ctl Alt Del together.

When the PC gives you a prompt, you can type 'format C:'.