You may have to reformat. Installing beta
software on mission critical
machines is generally not a good idea.
There is no published way to remove SP3 from a non-bootable system, but these instructions are for SP2 and they might work:http://support.microsoft.com/kb/875350
Because you have Media Center Edition, and therefore do not have a windows disk, you are probably out of luck, unless by some small chance you had installed the recovery console.
- My statements do not establish the official position of Microsoft Corporation, and are my own personal opinion. (But you already knew that, right?)