Before you go spending money, you should have a recovery partition from which you can boot a full version of the system with which the pc was originally set up (unless someone has deleted it!). To check if it still there:
Turn off the machine, switch back on and repeatedly press the F8 key until you see the "advanced boot option screen." Select "Repair Your Computer", Select your country. On the System Recovery Options Screen select Restore Application. the follow the onscreen instructions to run a "full Factory Recovery".