A very common issue with some computers is computer thrashing. What this means is that when certain programs are open you will hear the fan sound very loud because it is having to work harder. This can be for a number of reasons; some are easy to fix and some are more serious. There are a few things you can try to remedy this issue and have your computer running quiet again.
One way to limit some of the thrashing is to turn off Windows indexing. This is a program that makes it easier to find and search for programs on your system but if you pretty much know where things are at it is not needed and can free up some resources.
Adding more memory is another remedy that may work for your computer if it is thrashing a lot. By turning off auto-defrag in XP or scheduled defrag in Vista, this may also help out.
While none of these fixes may take care of your thrashing problems, they are helpful guidelines in knowing what to look for and what to try before you take your computer into a repair shop. Vista seems to have more issues with thrashing so it could be cleared up by going to a different system such as XP or Windows 7. This all depends on your personal preference but by downgrading or upgrading you may be able to clear up some of the thrashing that is going on and not attributed to a certain problem.