I recommend purchasing an IBM Thinkpad -
I am on my 4th ( T22) and I can honestly say I have never come across better build quality. A Thinkpad is not the most colourful laptop you will buy but buy a Dell or Sony and drop them once from a table and watch your money sail away.
I recommend the R Series ( R30) if your sticking to budget, its all in one with a decent 15 inch TFT screen, you have the choice of P4
or celeron models as well.
If you can afford the extra chas go for an A Series or T series -
either way if you buy a Thinkpad you will still be using it in 10 years time.
Hope this helps
Link would be handy -
http://www.pc.ibm.com/uk/thinkpad/