Whenever you are looking to upgrade a PC the thing that always gives you the most 'bang for your bucks' is more memory. If you are running XP with 128mb of RAM, no wonder it's crawling. Your suggestion of 512mb is good and you will be amazed at the difference it makes. At the moment your PC is having to use a portion of it's hard disk as 'virtual memory' in order to supplement the RAM. You've probably noticed your hard disk is thrashing about for minutes on end during the boot process.
To check what you can have in your two slots use the Crucial memory checker from here to tell you.
Upgrading your CPU is possible but it depends on your motherboard. Let's leave that for the moment until you've put in more RAM.
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Do you know that we're all in line for succession to the throne? Really?
Well, if forty-eight million, two hundred thousand, seven hundred and one people died I'd be Queen.