#1: Stop using the machine. Turn it off NOW, and leave it off. Seriously, I mean NOW.
#2: OK welcome back, glad you have another machine available to log onto MR with. The reason you need to turn the machine off is that OSX is continually creating and moving swap files, overwriting 'free' space on the drive, and every minute the machine is on decreases your chance of recovering your file.
#3: Purchase DataRescue II software. THis is the best on the market, but be advised that they specifically do not guarantee recovery of deleted files from an OSX volume (especially if it is the Startup drive) because of reason #2
#4: Get an external storage medium of some kind ready, an external Firewire drive is ideal, a USB drive or even a USB memory keychain thing will do in a pinch.
#5 Boot the machine from the DataRescue CD and perform a scan - this will take many minutes or a few hours. Look at the results of the scan and Recover the likely looking files to the external storage drive
#6 Next time, BACK UP your important data on a regular basis. Also, it is a very good idea to have an external, bootable Firewire drive large enough to mirror your system, and to make backups to.
#7: Welcome to MacRumors. Get friendly with the Search function on the site, because we have addressed this question many times, and it may be faster to get an answer by Searching rather than starting a new thread.