Here's a couple things that I've seen go bad on 5+ year old machines:
DUST -- open up the machine and look inside. If there's a bunch of dust everywhere, take it outside and blow it out with a can of compressed air, or a hair-dryer, or an Air-Compressor.
CPU Fan -- Make sure its turning nice and fast and pushing a lot of air. If its slow and noisy, or not spinning at all, you just found your problem!!
Drivers -- Make sure they are all up-to-date, especially the Video Driver (especially if its ATI -- the older versions of the ATI Video Drivers are among the worst dung-piles in all of computer history). Check Sound Card, Video Card, and Motherboard (ie: chipset) drivers.
Memory -- Memory can go bad too. Easiest way to test it is to download a bootable Linux CD (such as Knoppix or Ubuntu). Burn the .ISO file to a blank CD, then boot from it. MemTest86 will be one of your bootup options. Choose it. Let the memory test run overnight. In the morning see if there were any failures (if so, time to replace your memory).
Power Supply -- The one thing everyone always over-looks...
Make sure its fan is spinning and pushing a lot of air.
Make sure it is not all dusty inside (you need Compressed Air for this, a hair dryer or human breath won't cut it!).
Also, Power Supplies will start to put out "dirty power" after awhile (DC Ripple and/or Low Voltage under load, for all you Electronics Hobbyists and Engineers).
The easiest way to test if your Power Supply is the problem, is to connect a different Power Supply (from another computer, for instance) and see if the computer suddenly starts working normally. Unfortunately not everyone has a second computer they can swap Power Supplies with!
The other way to test it is only helpful to you if you are into Electronics... Simply connect a Volt Meter or Oscilloscope, power up the computer, and watch for any Voltage Drop or Ripple.
As for Vista, I wouldn't get it just yet. Wait a little longer for companies to make Drivers for it, and
Honestly, it will probably end up being cheaper to buy a new computer with Vista on it, than it will to buy a bunch of upgrades and a copy of Vista for your existing computer!!
If you decide to go with Vista, here are some things you'll want in a new computer:
At least 2 Gigs of Ram is an absolute must.
A Dual-Core CPU is an absolute must.
A good Video Card is also very Very VERY highly recommended... Not an absolute must, though.
Look for ATI or NVidia. AVOID the "built-in" or "onboard" Intel/SiS (or anything else with "shared memory architecture").
Without a good Video Card, you won't get any of the new "eye candy" features and some things are going to be a bit slow.