Is there a program that allows someone to remotely watch your computer screen over the Internet (e.g., watch a PowerPoint presentation)?

yes more than one. I have used pc anywhere its easy to use

LogMeIN, ShowMyPc

Slightly more complicated, but great cross-platform (and no client installation) is TightVNC -- requires a firewall rule on the hosting side, but anyone with the password can connect to the web server and get your screen.

The GoToMyPC.com people also make GoToMeeting.com and GoToWebinar.com --
I haven't used them , but the GoToMyPC is so easy to use, I'd imagine these are comparable.

I'll second the recommendation for GoToMeeting. I used it regularly in my past job and it was much more affordable than the other professsional packages out there (Webex, microsoft livemeeting, etc).

Logmein works fine for me

"Glance" is a good program. Pricey, but you can try it for free.