Here is my old pc specs... (i gave quite a few of my old parts...)
 
3.0 P4
512 MB ddr400
6600GT
120 IDE
good asus mobo
 
anyways... what could i use this pc for?  i hate throwing things away (like most people   )  Could this computer be a server - possibly for cs source? I ve never set one up...

I will use it for your file & backup server at home if I were you  
 
I don't know for CS source server, never tried that too ... But might be a good idea

put Ubuntu Linux, this will bring your pc back to life with very fast speeds.

Door stop, no just kidding.  Never throw working PC's away, donate them to someone who's in need like schools or a church.

If it was legal, I would put some C4 at the bottom of my old PC, detonate it, and see how high it would go.

Just send it to me. It's better than I have at the moment and it looks like my build has been put on hold AGAIN.

 
yeah it could be used for a server, the GPU not being very important in this case...however you will need mroe memory luckily ddr 512MB isn't very costly. i forgot what to google but there is alot of information as to running CS:S servers and some good graphs thyat show you how much CPU power u need [you are good], memory usuage i think 512MB willbe good for only 12-18 slot i think? and it also tells you about how much bandwitdh you wil need depending on how many slots youa re thinking about running.

Yes, that machine will be fine for running a CS:S server BUT it would need more RAM to ensure smooth play if you intend on setting it up for a large amount of players. I'd say you need 1GB if you're going to run the server in a windows environment. Other than the RAM issue, more important is your connection to the internet. UPSTREAM is extremely important to running a game server. If you're on the typical cable connection with lots of downstream and gimped upstream then you can forget it.

 
Seconded if you're going to get rid of it. Recycling it could also bring a little bit o'cash. Or so says Discovery Channel  
 
If you're going to keep it:
NAS
HTPC
Kid's machine
Spouse's machine
Visiting friend's machine

how do i configure it for server/backup though???  what would the advantages of having it as a regular old home server?

Folding at home.

i'm about to get a new pc my self.  The one i'm using now i'm just going to have for my partner to use to surf the net and do what ever she wants on it.  also for some of my old games that i have a feeling will never work properly on vista

-F@H - toms team is 40051(this can run in the background for all the others too)
-Server(you dont need anything that fast for most common server tasks.) Try CS and see how it goes....keep an eye on the memory use and then see how many users you can host for.
-NAS(fill it with hard drives and have access to it from any computer on the network, its still a server)
-download/torrent machine(just limmit its bandwidth or it will bandwidth whore the network)
-plays everything video player(the 6600 has tv out) and VLC works great(post processing also helps crappy files). If my celeron 800@1060 can do it(did not try post processing,but the cpu loads are low so i bet it would work)so can your p4(it just will not run on a 90 watt psu ).
-Emulation machine.(MAME,Snes,Nes,some n64). This can be added to the plays every video machine....
-Donate it. many people have FAR worse then that...

 
smoking something, yup.  

1. put it on craigslist to pay for your next tank of gas or video card
2. data server - plug into network, share a folder/drive
3. media center computer - upgrade cooling to make quiet and hook up to tv
4. folding@home - set up for remote desktop connection, start folding, unplug everything but network and power as they are not needed
5. obtain video camera and friend - then run over with beater truck and post video link here.
6. generational gaming.  fill it up with games from the last 5 years that work well on that system and goof around with it.   (i keep a p3 733 and voodoo 3 3000 lying around for just that)
7. Experimentation/testing station.  open it up to test stashes of parts, install drive partitioning and cloning software for easy repairs.  
8. Apply 120 volts live to various parts when system is running - video tape and again - post link here. Warning - likely smoke if not fire.