As stalion says, I would think it's hopeless. The motherboard is unlikely to take any processor available today. It probably also has EDO memory which is now hard to come by. You would have to replace everything. In other words a new pc. Sorry, but that's how it is. Everything has moved on.
Seven years old would probably be no more than a p166 with a max of 64mb ram.
I have upgraded 3 'older type' computers, and to be honest "It just wasn't worth it".
Seeing the price of modern computers with the latest components, just doesn't make it worth while to pay to upgrade on major items. Plus with the new computer, there is a warranty for at least twelve months, covering the whole system.
..I upgrade old ones for charitable causes constantly. One of your biggest considerations needs to be the memory, you need 128mb to load and run XP alone, but preferably 264mb. If your motherboard can take this, then fine. Can I assume that you are running at least a P2? It will do the job for office working and internet but to dampen your enthusiasm slightly, you won't be able to play any fast games.
You will need a minimum of 3GB HDD to store XP and the basics such as Office, a firewall, AV etc.
So..it is not as hopeless as some think provided your system is not entirely too old for the job! Feel free to mail me if you need further help...
I occasionally use a 266, 64mb computer, 8 years old that has been upgtraded to XP. It is fine, if somewhat slow but it was slow with 95 on it. All this 'tis nigh impossible to upgrade unless you haev a gazillion Mbs', is somewhat premature.
Hang in there mate, Last September my niece gave me her old computer a Brother with W95 on board, a 300 MHz CPU. 64 MB RAM and a 2 GB HDD. I immediately loaded W98, no problems and having looked at a mates XP Pro looked around for another Motherboard. A Dell Optiflex G1 came my way and after an increase of CPU and RAM I loaded XP Pro seamlessly and have still got it running sweetly. Only problem , after running off so much, I don`t Know how old my Motherboard is. But it has got wrinkles.