Not to be a smart***, but, get a new machine.
I'm serious -- even if you're on a tight budget, it's not hard to find a
computer (with monitor -- sometimes even LCD) for just a few hundred bucks.
It won't be top-of-the-line, of course -- but still a big improvement.
If you don't want to go that route, and are tech-savvy, start upgrading your
machine in steps. That's what I did. My machine is a home-brew, and rather
than sit on the hardware and allow it to become outdated, I've started
upgrading a component 2-3 times a year. I aim for "middle of the road"
performance. Watch the sales, take a look at eBay (don't be afraid to buy
used -- but make sure the feedback is VERY good), Craigslist, and even your
local "forsale" newsgroup. Heck -- you can even sell your old components
back using the same resources, and make some of the money back -- keeping
the cost even lower. If you have Fry's nearby, they offer CPU/motherboard
combos almost weekly. It's not hard to find a pretty good CPU/EDS
motherboard (not the best but still an upgrade) for around $100. RAM is
cheap these days -- especially for a desktop, and so are hard drives. I saw
a 200 gig HD just recently for $59!
If you don't want to deal with rebates (and who does?), keep your eyes open.
Deals can be had w/o rebates -- they're just less common. Still -- all the
prices I listed above were OTD prices -- no rebates involved.