Hi Peter
Its only running programs that cause the problem, virus checkers can also be resource hungry, Windows Media Player most definitely is.
I had similar problems to you with my Tower System, when I was running Cubase VST and Media Player, and which increasing my Ram from 512Mb to 1Gb cured.
As a test restart your computer, and when Windows has loaded, press Ctrl-ALT-Delete to bring up the Task Manager, Select the Performance Tab and it will show you the Physical Memory being used and Page File being used.
Start your programs up and use as normal and just keep an eye on how much the available memory is being used, as well as the page file, this will give you a good indication as to how much more memory you will need to run the programs. (The page file can be considered as additional Ram)
As an additional point, some programs are very poor in how they use memory, so while in 99% of cases extra Ram helps; it is not a 100% guarantee.
Hope this helps.

Bill