Hacktivate

Installing Windows (XP, 2000 whatever) under Ubuntu:

Install vmplayer:
sudo apt-get install vmware-player

Make a directory for windows and change into it:
mkdir windows;cd windows

Create an empty vmplayer disk image:
qemu-img create -f vmdk windows.vmdk 2G Formatting 'windows.vmdk',
fmt=vmdk, size=2097152 kB

Put a vmplayer configuration file in the directory. Get mine by running
this command:
wget http://www.vdomck.org/~crazy/windows.vmx

Put your windows CD in the cdrom drive. (See configuration file for
dealing with iso files instead of CDs)
Run it:
vmplayer Windows.vmx

update
Some people are having trouble finding vmplayer in their repositories. I think I know why. After I installed Dapper, I edited my sources.list file and made sure restricted, universe, mulitverse were enabled by uncommenting the entries that were helpfully placed there. But it wasn't until I used the GUI (Synaptic Package Manager->Settings->Repositories) and edited the first Ubuntu entry to include main, restricted, universe and multiverse that I was able to get everything I was supposed to be able to get. This caused the first line in sources.list to look like this:

deb http://us.archive.ubuntu.com/ubuntu/ dapper main restricted universe multiverse

So if you are getting "Couldn't find package vmplayer" or anything else that you think belongs in one of the Dapper repositories - try changing to the above.
updated update
Looks like I just had the package name wrong. It is vmware-player not vmplayer!