Hi, I had the same issues with SP3 breaking my MCE2005 but you don't need to do a fresh install. There are a couple of articles on the web about what you need to do - one of them is
http://forum.aumha.org/viewtopic.php?f=62&p=188111.
Apparently SP3 breaks some .NET modules, and somehow the basic MCE 2005 processes handling live TV/Guide no longer register and hence give you the above errors. The basic process is
1 - download and reinstall .NET
2 - re-register the MCE 2005 by running medctrro.exe
3 - download and reinstall any updated graphics card drivers as SP3 breaks a few existing ones such as my ATI
4 - get Terminal Services working again
Step 2 didn't quite work perfectly for me but I managed to get the key process ehrecvr and ehsched by going into c:\windows\ehome and clicking on a "register" exe. I can't rememer exactly which one it was though and you may not need to do it. Anyway there are more detailed instructions and explanation around - such as Aaron Stebner's log.
Step 4 - I also had to redo the Terminal Services patch to enable multiple users on the HTPC. Google "mce 2005 terminal services" to find the appropriate DLL and the instructions to replace this on your machine. You will need to boot in safe mode to replace the DLL and then reboot back into normal mode.