first of all, this might belong in the Tech forum. im not sure though.
second, there are a few possible culprits. your internet connection might be cruddy, you might have a program running in the background hogging some of the ram, there could be a vista issue, vista itself could be hogging some of the ram (it is in fact a ram hog), or one of many other solutions.
do the ctrl-alt-del thing, go to the performance tab, and check PF usage. dont worry about the number blow the chart, just look at the chart. if its showing that something is using a large amount of your PF, then check your processes tab. look under mem usage for each active process and see if anything is rediculously high (like over 100,000 K). if thats all fine, then it very well could just be vista.
also, whats the status of your internet connection? do you have anything like frostwire or limewire or ares open? those are some major bandwidth hogs and cause games to lag a lot if they are open. also what kind of internet connection do you have?
i would say the CPU might be choking on it but i doubt it. i used to play BF2 on a pentium 4 with no HT at around 2.9GHz which for a P4 isnt very much and i didnt have any major issues related to it. but to be safe, maybe try overclocking the CPU (if you think you can... its not very hard). for help on that,
this thread over at TechReport should help. but again, i highly doubt the CPU is the cluprit.