This is from the 'Ask Jack' blog, Technology section of the Guardian. He generally has pretty solid advice in easy to understand terms.
http://www.guardian.co.uk/technology/askjack/2009/nov/04/ask-jack-05-novemberOld PC is slowing fast
I am using a Dell Dimension 4700, which has become sluggish in operation and occasionally freezes. I understand from our friend Google that it could be a registry problem and that there are programs available to help.
Brian Thomas
Jack Schofield replies: The Dell Dimension 4700 was launched in 2004 but its 3GHz Pentium is more than fast enough to run today's Windows XP SP3, if it has enough memory. At launch, XP ran in 128MB, so the Dell's 512MB was generous. Today, the practical minimum is 1GB, so you may need to add more. Crucial's website has a system scanner that will advise you – and sell you the memory. Running out of hard drive space has an even more catastrophic effect, so make sure you have at least 1GB free – not counting the Windows swap file, pagefile.sys.
Otherwise, your PC's memory has probably been eaten up by big-name companies who want to make their programs launch faster, or want to keep updating them for you, but have little thought for what happens when everybody pulls the same selfish tricks. Unfortunately, there is no non-technical way to stop this, but Mike Lin's free Startup Control Panel provides a simple way to block the worst offenders.
I generally don't recommend that inexperienced users run cleanup suites because the results can be catastrophic, and I don't believe registry cleaners make any practical difference in most circumstances. However, CCleaner – which is only available in a free version – is one of the safer options, and it includes a registry cleaner.
The best solution is to reinstall Windows XP from scratch on a clean hard drive. However, with such an old PC, this will require a long series of Windows updates, and you will also have to reinstall your applications and transfer all your data. If you take that route – or pay someone to do it– then you may as well run the Windows 7 Upgrade Advisor. The 4700 isn't listed as compatible, but I'd expect it to work if you installed a new graphics card – look for one that supports DX11. Of course, if you have to buy more memory, a new graphics card and a copy of Windows 7, you'd be better off buying a new PC …