K well though I cant base your problem on a singular fault Ill make an effort.
You have low ram and windows XP is ram hungry, though 256 is sufficient is just that. What is more than likely happening even though no tasks are being your OS is being run and yes that uses ram also. NAV 2k5 and NPF are sort of hungry also, thats why many developers choose not to have them installed. I would advise (1) a lighter AV (maybe AVG from grisoft) light, reliable and frequent updates.
There is thing called virtual memory, which is used as a method to increase your ram. It doesnt really do that but it uses your HDD as a temp storage for the memory. In the case of opening an application that requires more ram thany ou have physically it will store some of these on your harddrive and them swap them into memory when needed. Since your ram is low, this is going to be utilized more often. Suggestion (2) defrag if your HD head is moving like crazy trying to get a sector here a sector there, a defrag should put them all in 1 nice cluster.
I noticed that you were considering purcahsing some SD-RAM rated @ pc 133 (yes ppl i know ddr is sd ram also) is this the type of ram you got? If so thats some old stuff, though I wouldnt put down the ram type, its old but can do the task and i wouldnt count it as being a significant cause of your problem. However the ram QUANTITY is a problem, suggestion (3) please buy that 512 stick it will help you out alot.
When last have you formatted? suggestion (4) If push comes to shove do a nice total format of your machine, its like a purge for a PC. It is recommended to format your machine every now and then, the actual time frame is based on usage.
Please watch out for bittorrent, its the #1 cause for defragmentation, if you should use it configure it properly and you should experience less fragmentation.
