If it is gradual, then you have what is called a memory leak associated w/ a certain process/program. Use task Mgr, look at the processes as stated before for those using the CPU currently. It takes a while using a monitor system (after cleaning out your Windows startup folder properly). Then you turn on 1 application/program at a time and use it noting differences until you isolate the program causing the memory leakage. A lot of what happens is related to software programming errors that can be repaired or patched by a software vendor, but Windows has always had memory leakage problems, most being caused because RAM usage is set aside in blocks as soon as you click to open a specific program, called Ram allocation, sometimes you open a program, Ram get's allocated to be used by that program only, then you realize that you really meant to open a different program and close the program before 90 seconds from opening it has elapsed. Then you open another program. That Ram set aside doesn't release, and just sits there leaking away. The simple cure there, if you open a program accidentally, wait 90 seconds or more before closing it and that Ram will return to being allocatable to other programs more easily upon closure of the "accidentally opened" program.
RAM leakage problems take time and effort to really pinpoint but can be done easily while you do other things, then you simply contact the program vendor and ask them to fix the bug....But you must participate in finding the offending program. A decent fix is to install another hard drive, set up your swap file (maximum size of that swap file should be MAX twice the size of actual physical installed Memory/RAM). IE: Switch swap file to another drive, size the swap file 2GB Installed RAM, 4 GB max-1GB installed RAM=2GB max.
Then you can monitor the swap of data and pinpoint leakage easily while increasing avail memory up to 3 times what you have physically installed.
Right now is Windows managing your swap file? Both XP and Vista will run @ 100% CPU usage as System Idle Process is a true process, but it shouldn't show under Task Manager "performance tab" as running at 100% (that is unless file indexing is running which is a pure memory hog)...which is what Vista's Superduper extra special waste of space and memory Aero Video does as is.
My suggestion...Downgrade back to XP, or simply set up a dual boot to XP system (easy enough to do), until Vista is truly ready for prime time would be my suggestion.
Need further help, e-mail me @ excite.com (same name as here), I'll try to help.