processor: go for about 3GHz Intel processor, or a similar AMD 64-bit processor (3000+, 3200+, whatever). basically, AMD processors run more efficiently than their Intel counterparts, so while the processor I am using now runs at 2GHz, it is called the 3000+, as it runs at an equivalent to an Intel 3GHz.
RAM: do you want to play games, do video editing, something like that? If so, go for 1GB RAM, maybe more. If you just want to surf the web, listen to music, do all the usual family things, and the odd game too, just go for 512MB RAM. some can be bought with 256MB, but in my opinion this isnt enough to get winXP running nice and fast.
hard drive: depends how many people are using the computer, and what you're installing. for the average use, 80GB should be fine. if you have a few kids, each of which has loads of their music they want storing on the drive, then go for something larger. letters, essays, that kind of thing don't take up much room. but lots of music can take a sizeable chunk, and video files (films) can take loads and loads.
also check out the graphics card. make sure it has its own one, an nVidia or ATI compatible one.
and dont be tempted to one PC by what software is included. if you'll be online, you can get any information for free from there anyway, and most other software tends to be stuff you'll never use anyway.