I have used that argument with laptops, but they reason that I'll be home most weekends to see the girlfriend (lives in same area, and is 1 year younger - waits for

He's a paedo

) so laptop won't be stuck there over the weekends. Naive imo, but that's their (own certainly my mums viewpoint).
I know this is slightly delayed but yeah....
I'm in my first year, and I don't need my laptop for the course in lectures - doing economics, judging by the fact you're doing chemistry, you most likely won't either. I did a term with only a laptop, and got fed up in the end and brought up my tower... I haven't looked back....
As for stealing stuff, walking into a room and picking up a laptop, is MUCH easier than walking into a room and picking up a tower.
Also, watch out for their expectations for you going home every weekend, you might well be, but don't have them expecting you too, otherwise they're going to gain some kind of clingy element, and will be demanding you come home every weekend - I've seen it in a few people, it truly is tragic....
In addition, if you get an agreement to negotiate after exams, make sure you get some kind of guarantee out of them that if you do X you get the computer... otherwise I'm sure they'll come up with an excuse of some kind!
Also, Stulid you're a lightweight
Best of luck
kd