My computer will start, but it has a problem with memory and will not boot.?

My computer is a dell Inspiron 1200 windows xp, and when i log on, all it shows is the background screen, but no start bar or programs. i have a nagging feeling it's because i dont have memory (>1gb) but i cant be sure... any help? i am running a diagnostics check right now, and it isnt showing anything. i tried to contact dell, but my warranty has expired.