Well I've just learned how lazy windows has made me! Windows doesn't really care so much about what ram is installed in motherboard to a certain extent, as long as slowest ram is installed first it will assume all ram is the same. Not so with ubuntu! All ram must be same type, speed and brand for this operating system.
I had 4 gig of ram installed in my custom comp. It now has 2. All the ram was of the same brand but different speeds. DDR 400 and DDR 466. After clean install of ubuntu 12.04, I went to update and alot of the updates didn't install correctly and I kept getting alot of errors for alot of different things.
I was practically pulling my hair because I had never had trouble with ealier versions of ubuntu. But,then I remembered after my ubuntu drive died; I went and purchased 2 more gig of ram for the xp I was running. That's when It hit me. Oops, windows makes people lazy. Not good I say!
So for those beginners with custom built computers and having errors check that speed of the ram, very important.