I've thought about the same thing when people ask, but I don't worry about it anymore.
A new lass has been working at my place now and I've been chatting to her in passing. She heard I was at uni and asked what I'm studying, so I just told her it's a 3 year computing science degree, been into computers since a young age and have been interested in them ever since.
"Ooh computing", she says. "I'm doing a basic computing course at 6th form"
Now I know exactly what she's talking about, so I say, "oh yeah I found that stuff really dull." She agrees.
I then go on to talk about programming, at this point I thought I was going to lose her now, but she said "Ooh wow! programming. There's a lot of money in that. Is it games programming?"
"Nah", I say. "I'm not good enough at maths, don't think I could manage that kind of stuff"
"I actually enjoy maths and am quite good at it some stuff", she says.
Since she seems to be interested I go on to tell her other stuff about the course.
She's fairly attractive too, and managed to keep her interested until I finally decided to change the subject myself.
I always talk about what I'm studying with enthusiasm and show that I enjoy it. As long as you don't talk about complicated things that they won't understand and are actually boring, they usually don't twist their face.
It's nothing to be ashamed of, much better than a degree in history :\
If the person doesn't know computers and is just asking to be polite, I won't talk about specifics, I'll just mention what's involved, what kind of job I could end up with, the money and I'll talk about projects I've done if they still show interest. And, of course how often I get wasted on nights out between studying and tell them a funny story from my last night out, will always bring them round if they're dosing off