Do you have a floppy drive and are you willing to format the Pc if thats what it takes?? Also is the OS on a bootable cd that you plan to install?? If it is yes to all of these I found a trick that I use....I boot up with the floppy or the bootable cd and do a fdisk and then I delete the partition and then it allows me to format drive and reinstall whatever Os I want.
Also, you said you have a compaq...do you have a d drive that maybe has the restore windows on it?? My dh has this on his compaq and you can restore everything to factory settings that way.
Hope I did not confuse you.