If you have done a full install of XP and didnt have your drive partitioned so that you only installed to C drive then it will have asked you to format just after where it asked you where you wanted to install XP to.
In that case you have presumably wiped all your data.
If thats the case do not do anything else with that PC for now. I assume that you are using a second PC to post on here, hopefully.
There are certain programs that you can boot your pc from and that may, if you are lucky, find some of your old data that hasnt yet been over written. You would need a second hard drive or an external one to save any recovered data to though, as if you tried to recover to the same drive then it could overwrite what you are trying to recover.
One of the best programs I have found for recovering data is File Scavenger. There are others and it may be worth trying a few as some may recover something that one cant. You probably wont get everything back but you may get a lot back if you are lucky and patient.
Depending on the size of the drive and the amount of data you had on it can take a while to find and recover it.
Get back to us if you need any more help, its a pity that your first post has to be such desperate one but we will do all we can to help.