As per JiKs suggestions, but I personally recently went for a laptop (with Win 7) - the difference in price for a laptop vs a simple desktop, that usually comes complete with monitor, keyboard, mouse, etc, the effective price difference was negligible, and the speed upgrade was noticeable.

The only hassle may be the screen - if you have an old small CRT (tube type), you can get 22-23 inch LCD monitors, and even similar HD TVs that will work well as a monitor for not much, if you don't want to use the existing monitor. Especially if you get one of those cheaper laptops with a 10 inch screen...

Also the laptop Keyboard may not be as well liked, so you can just plug in your existing keyboard & mouse for familiarity.

For relatively little money, you also can get a KVM gadget that allow you to use the same monitor & Keyboard & mouse on both machines, and switch between them.