You probably will need to reformat your HD. I use a NimbleX Linux disc for backing up my files before a reformat, as it lets me access the computer and my HD without installing anything.
It runs straight from the CD and you can use the Internet and USB devices with it.
I use the Sub 100MB version. click here to download the ISO file. You will need a program capable of burning ISOs to CDs, so click here for CDBurnerXP Pro.