How can I encrypt my computer files?

Bitlocker is plenty for most people.  But you're right, it doesn't come with Win7 Pro.  You'll have to get Ultimate or Enterprise.

Anything third-party?

TrueCrypt is also my encryption utility of choice. Open source, good security practices employed and it is clearly not going away any time soon.

TrueCrypt can encrypt entire volumes - or you can just create a single "container file" which TrueCrypt can mount as a virtual volume. I personally prefer the latter.

Truecrypt is also my preference.  Coming from the accounting industry, this is the program that I used for all staff working on Fiduciary accounting and this was withing regulatory compliance according to the US Government, actually the same program that they recommend for encrypting system drives.

Thank you.  I know that other people recommended the same, but to quote the lyrics of Nelly, "Two is not a winner and three nobody remembers."

Just keep in mind that you should use regular backups from encrypted files. If by any chance you forget the password of have technical difficulties to decrypt the volume, all data is lost and there is no linux bootdisk to grant access to the files (in contrast to lost windows passwords)

For details see also:

http://www.truecrypt.org/docs/?s=rescue-disk