Like, rip out the junk, stick in something better, get a new duo core cpu to go with it and then attatch it to the hard drive that i used with it before, so i dont have to get a new OS disk? Like okay, I was thinking about replacing my Dell Dimension 3000 with a self build, less suck computer, but that means id need a new OS Disk, and new other shit that im not worried about replacing. What Im saying is, can I rip out the mobo, ram, processor, the thing came with, put in a sexy new mobo, ram, processor, and gfx card and plug it into the hard drive thats allredy there with the OS installed?
If you are running winXP you can do as you have said so far, then put the XP cd as if you were going to install, but instead choose to do a repair install, a good guide for doing this is here. This will mean that after you have done the repair your OS will seem exatly the same although on new hardware. You invariable have to do this whenever changing motherboard unless it has the same chipset as the previous one (as far as I have found). Hope this helps
One thing that you should be aware of is the Dell PSU wiring job. While they use an ATX formfactor, they wire the main ATX power block, and corresponding motherboard connector up a little differently. While there are a few that don't, if you get a non-Dell motherboard, you'll want to factor in a new non-Dell PSU to accompany the purchase. Yes, it sucks, but frying your new parts sucks even more.