Need to build a new system, tight budget.

Hi guys, just signed up.I need some help please.

My little brother just screwed up his 4 years old HP computer. We get a "NTLDR is missing" error and tried whatever I culd but I will need the windows XP disc to get it working again for the other solutions. But I can justifie enough to buy a OS so I want to build a new computer using some of the parts fro the old HP , if possible.

Here are the specs:
-AMD Athlon(64) XP 3200+ running at 2.0 GHz
-GeForce4 FX5200XT (Lancer) 128mb agp 8x
-512 ddr ram
-200gb HD seagate
-Im not sure, I guess its a socket 754 mobo
http://h10025.www1.hp.com/ewfrf/wc [...] uct=404646

The thing is I cant go over 500$ os included.

I didnt list parts already because I know people would post better things. I do have a thinking going on though.
Instead ill list what my bro usually do. He is casual gamer, like Age of empires 3 sometimes, on low and usually on the net or play free online games.
I think I like Intel better but its more expensive that way right?

Oh and im in Canada and will be buying my parts from NCIX, but thats not really a problem because I dont think the price differences are that huge. At most ill get my Mom to help me out...
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Just a side question, where is everybody getting there OS with system buildups, nobody lists it in theyre budget? Can you just reuse the CD numerus times or what? Am I missing something here??

Thanks guys

Canadian prices aren't bad, if you know where to shop.