Best OS Font?

My Windows 7 install could use some sprucing up and I figured I'd start with the font first. I like Segeo UI, but I've been kinda wondering if there was something better over the horizon. So I was wondering, what font do you guys use, if any, for your OS? The obvious criteria I'm looking for are: - Scalability - Looks good, correct spacing, ect. on any size - Readability - Easy to read, crisp letters - Open Source / Free - I'm a high school student without a job so....