Well the up side of that is that you can make a difference for not a lot of money.
I'm going to assume that it still has the default 512Mb of memory, and to upgrade that to 2Gb of 240-pin DIMM, DDR2 PC2-5300 would cost approx £35 (price quoted from
www.crucial.com), which should make quite a difference.
A complete reinstall of the operating system should also get rid of anything that's causing it to lag badly, especially if it still has the original Norton AV installed.
Basically getting someone (ie man in small shop) to do a refresh and RAM upgrade, should cost around £100-150 at the most, as long as you back up your own files before hand, and should give you a significantly faster machine.
If you want to get a new shiny one then £350-500 should get you a decent new one from either PCWorld or even Dell direct.
If you are going to use PCW always check their website for prices first, as they can equal quite a saving on paying instore. By all means look instore, but them either order home delivery or collect in store from their site. If you have someone handy who knows what they are looking at their refurb offers can be OK as well; just don't be tempted to buy a Packard Bell no matter how good it seems!