Well the thing is anything other then a full format can still leave traces of unwanted stuff on pc,a format totally erases all and you really do have a new pc in effect.
Also remember if you have one pc then it's not cheap-a format is totally free too.
Graham if you can give me the type of O/S you are using i am sure i can send you a windows disc to install from.
If possible check underneath or on the side of pc to confirm you have the original serial number as we will need that to Validate your copy.If not we can use
http://www.magicaljellybean.com/keyfinder/to get the serial number.(if you download this do custom install and uncheck toolbar options)
I'm guessing you probably have windows home edition 32bit?
I can also supply you with the program to find your drivers too as that's often the hardest part.