Yes, you should be able to use a lot of existing components from your
current computer to build a new one. You’ll need to buy a new motherboard,
processor, and memory at the least. Depending on how old the computer is you
may also need to buy a new power supply, and you may want to buy a new
graphics card.

If you don’t need to play the latest video games, you can get a really
speedy video card for less than $100. 4 gigs of ram should be less than $100
as well.

I’d check out newegg.com for parts – great place with good bare-bones
combo deals.