What can i buy to speed my computer up?

My computer is usually pretty fast but lately it has been slow. It takes a while to download pages and watching videos is impossible.

I have an HP Pavilion 513n with windows xp and it's SOOOOOO SLOW! I want to speed it up and everything on it is useless junk, because we have a new computer, but I don't want to install the sims 2 on mt new computer. will completely deleting everything on the old computer speed it up much? If so, any tips on how to delete everything?

After downloading IMVU, my computer started running very slow. After a while, i deleted it from my computer, and my computer is running slower and worse than ever. Things keep installing themselves, and at random times things from internet explorer like advertisements keep popping up. It freezes very often as well. I've tried deleting things that arent necessary on my computer, and its only improved a little bit. What can i do to fix this my computer?

I use an Compaq Presario SR1103WM when I'm at my Dad's house. We've had it for about 2 or 3 years, and used it for everything from games to internet to whatever you can think of. However, it's outdated now, but I can't afford any major upgrades or to buy a new computer right now, so I'm trying to figure out how to run it better. The only specs I know are:

248 RAM (8mb is diverted to video memory) max is 1 GB
Intel Celeron(P) 325 2.56 GHz
40 GB HD

I have no clue about what the video card is, I've had it 3 years and still can't figure it out. Sorry the specs aren't very informative, but you could probably Google the rest if you need it. Right now it's on a dial-up connection, which I only use for instant messaging. Like I said, as of right now I can't really afford to upgrade it, but here in about a month I can. We're also getting Road Runner for it, which should speed up the internet connection a bit. Besides upgrading it, what else could I do to make it faster?
Also, I have deleted a lot of old software that I don't ever use anymore, and I just figured out how to clean the hard drive, so I did that for the first time in 3 years and it is sped up slightly.