My family is finally letting me upgrade the old desktop in the house.So i came hi looking for some advice. now i know how to build a computer and replace pieces because they taught us in school. what they didn't explain was when you are upgrading what to look for in parts because learning basic java and c++ functions were more important.

right now this are the things i want to change.
windows xp
processor intel pentium 4 cpu 2.53GHz
Memory 1022mb

I understand that if i change my processor i might also change my motherboard.

what i want the computer to just run windows 7 smoothly when i change the os.

also if you think it would be more cost effective to buy a new tower let me know.

You're better off buying a new computer (either desktop or laptop). It's not worth upgrading what you have right now, because you'll have to upgrade basically everything except the case itself (and maybe the PSU and the CD/DVD drive if it's SATA)

A just ruler amongst tyrants

Might even need to upgrade the case to fit a bigger mobo.

Just make sure all the parts are compatible with each other.

By the sound of it, and from personal experience, you might be able to keep the case, PSU and hard drives for the new system. But if you're not comfortable with hacking apart your case for adequate cooling you might want a brand new case as well... And your old PSU might also be too old, not right contacts and/or underpowered...

All of my own systems have been built up from spare parts and old cases, and buying new parts has always been a bit of a headache with deciding how much I want or can spend; just realize that chasing the bleeding edge is a game left for those with more money than sense.

Some computer shops sell entire systems in parts, and upgrade kits, which would guarantee compatibility between those parts if you don't feel confident with picking yourself.

The very last option to consider is a complete pre-assembled PC.

Maybe you could start with these Newegg videos.

Generally what lambchopsil said. Better to just get a new PC then messing with upgrades especially if the only experience you have with assembling and replacing parts has been in a classroom environment.

If you're dead set on trying to assemble a PC on your own another good idea might be to write up the specs you want. Then go around the shops and ask them to assemble a PC for those specs. Just make sure to tell them that you want info and price for each part not the complete PC. That way you'll have an idea of the prices and you can also check if there is something in your current PC that can be reused.