It won't hurt to increase the RAM but it will probably not increase the machine's slow response - you don't say how much RAM you have...
Check the startup list and uncheck everything except any security related items (RUN msconfig OK)
Download Spybot Search & Destroy (free) and run it - run it monthly or even weekly.
Download Piriform's CCleaner (also free) and use its registry feature to tidy up your registry
Defragment your disk - anually
Use Task Manager to see what's going on... Users - Applications - Processes - Network - see what's eating the machine's resources.
Get rid of any dodgy software you may have
Over time more and more naughty software gets into machines and makes things run in the background sending marketing information out to unscrupulous companies for instance - the naughty software is not always reliable (why should they care?).
Then think about adding more RAM - maybe...
After all - it used to run quite quickly when it was new and you probably haven't taken any RAM out I would guess and the RAM you have hasn't stopped working so you very likely don't need any more...
Good luck...
Peccavi, November 2009
