lppier,
What kind of PC speakers are you using? The 2.1 satellite and subwoofer type or just left and right speakers? If you are using the former, I'd suggest you watch the subwoofer level and take care not to turn it up too much.
There are a few things that might have affected your mix. Firstly, how does it sound on other sources (headphone, earphone, discman etc)? Compare it between different sources and try to get a general idea of how it sounds. Does it lack bass overall? Or only in a certain listening condition? The next thing is your friend's sound system? Are you familiar with it? Do you often use it as a reference point for your mixes?