Completely Erasing Photoshop?

ok guys, i bought a laptop off my friend who had photoshop cs3 and had either a trial version or illegal version i dunno, but anyways, i wanted to put a my version, student version on it, but it keeps not allowing me to enter a new serial number in, even if i uninstall it, and remove all the common files etc. i read about deleting the shared file cache.db but that still didnt work, someone please help me to whipe it out so i can add photoshop new. basically so photoshop never knows there was another version on here before

Completely Erasing Photoshop?firewall

Try uninstalling it completely, then running a registry tool like Registry Mechanic. Those tools usually remove the stuff that a program forgets to delete. Or, you could search through your registry (via Run... regedit) and use the Find feature to manually delete them. They are usually listed under the Program name, or the software developer's name. Or, if you want, run a Yahoo Search for CCleaner, download it, and run it. You can remove temporary files, un-needed registry entries, and tons of crap that gets left behind. the first C, in fact, stands for Crap, and it really does what it says, Cleans the Crap out of your system!