If you're buying a new computer then definitely get one with Vista, Service Pack 1.
Get a dual core or a quad core processor, 2.5GHz, 2-4GB RAM.
If it's a desktop, make sure there are plenty of USB ports. Some PCs have some on the back for things that are permanently attached, e.g. keyboard, mouse, printer and some on the front for memory sticks etc. If you use a digital camera, think about a computer with a built-in memory card reader.
The flat screen monitor should be 17"-22".
On the other hand, if it's a laptop, think about how long the battery charge lasts, the weight, and the size.