When I want to capture a computer screen, I use PrintKey. Version 5.10 is freeware, and you can snaffle a copy here.
Safe to use, I know several large corporations which use it.
The (now) commercial version is available from here.
Thanks goes to this site for reminding me to blog it.
September 2008 update
Do not believe this:
Printkey 2000 will not work with XP, Vista or Windows2003.
It does work, I used PrintKey 2000 regularly through the Longhorn client -> Vista Beta -> Vista SP1 test cycle, and it just worked.
For other screen capturing utilities, see this post.