Hiya Andy,
Inspirons are a dead easy laptop to work on and modify (I've got three 1545's). The main drawback is the quality of their HD's - if you're really unlucky it's a Hitachi, which is guaranteed until first use. Whilst it's a fairly obvious question, have you tried another HD? Also, if you're still running Vista, please consider changing to an OS that works properly, e.g. Windows 7.
If you find that your HD actually is to blame and you're feeling particularly brave, you might like to try a 128GiB Kingston SSD. They're currently selling on the right side of 50 quid at
ebuyer.com. My laptop now goes like a very nippy thing indeed, what with the new SSD and upgrading the CPU to a T9300.