Have you tried doing a clean install as mentioned in my wiki below. Also if you are going to be performing a clean install now may be the perfect time for you to upgrade to Windows 7.

Thanks for the help but my computer wouldn't let me do anything, even with the windows disc so I was unable to do a clean install or system restore. Anyway a couple of days later it started working again...for 2 weeks (thankfully giving me time to take everything off my hard drive)..then it failed to load again. This time a diagnostics test revealed that the hard drive had failed.

So I have taken it into a shop. I asked if I could get it upgraded to Windows 7 while its there and they said that it would cost loads to do this and therefore wasn't worth it

is this right? Can't an Inspiron 1520 be easily upgraded to Window 7?

Well you need to first buy a 2.5" sata laptop hard drive probably 500 GB or  so (you need this irrespective), unfortunately these have went up in price due to flooding at a major factory, secondly you need to purchase Windows 7 upgrade license, these can be a bit costly but if you are student you might get it for a severely discounted price.

You can buy the hard drive direct from Dell or amazon or eBay but I recommend your local computer shop. However look at the prices and whether they are in stock before purchasing.