If you are receiving this message when attempting to install the Apex Tool kit, then you are probably using Windows Vista Service Pack 1 or earlier and you will need to bring your .NET frame work up to current.
Use the link above to grab the most recent installation of the .NET Framework.
After installing .NET, reboot your machine again and try reinstalling the Apex toolkit.
As a last ditch effort, you may attempt the following yourself, but I recomend that you contact our help desk and allow us to remote into your machine and do it for you, just to be sure nothing else is over looked.
Add the value of OnlyUseLatestCLR with the value of 1 to the registry under the following key:
(that string must be added w/ the value of 1, because its usually just not there at all)