The speed shouldn't slow unless your machine picked up a PARASITE. Read http://www.doxdesk.com/parasite/
And yes, 256 to 512M of RAM will help, but only if there are no pests.
Bob
I have a Dell Dimension XPSB 800. It has an 800mhz processor and 128mb of RDRAM. It is running Windows XP and has been since XP was released. The computer itself is becoming increasingly slow and many times, programs will crash (especially Internet Explorer.) The slowness is killing me and I need some advice. Do you recommend purchasing a new computer or will an upgrade of the memory to 256mb or 512mb solve the problem.
Thanks for your help.

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