how do i restore my pc to new?

ive got a packard bell pc which ive had for a couple of years but it is now very slow, freezes up quite often and just generally annoying is there any way of restoring it to its original state, ive tried system restore but it says file missing and i have no recovery disk of any sort, if i was to delete everything off manually and reinstalled again would this work or just make matters worse any advice would be much appreciated thanks