Error code 1046: new hard drive, still problems

I have a Dell Inspiron 2200 laptop that gradually died. Before it went completely kaput I ran a diagnostics test and got:

DTS Short Test Fail
Error Code 1000-0146
Unit 0: DST log contains previous errors


A few of the breakdown symptoms:

[All of these symptoms were progressive. They began intermttently, and gradually became permanent. They did not start all at once, but rather one after another.]

Numbers did not work (except for 5&6 and characters)
Some letters stopped working (but not all)
DVD would not play but CD would.
Internet connection could not be detected.


Tried to reinstall windows.
Would not allow me to reinstall XP (after deleting partitions). Could not get past 40% of files loaded.

So OK- I REPLACED THE HARD DRIVE

XP would not install off DVD.(hmm) But it seemed ok with a CD. Thought maybe a bad DVD. Got install going ok until I came to part where you have to enter product registration code. Found out numbers still don't work (except 5&6 and characters) (more hmmm). [Incidentally, I tested the DVD on another machine and it is ok.]

Any ideas? CPU? Memory? Motherboard?

Any help is greatly appreciated!