Here are the steps from
http://www.tss2000.nl/vbforum/showthread.php?t=50 :
DLL patching hero Rafael has found a great way to enable your MCE 2005
install to join Windows domains. Rafael, known for patching the now famous
uxtheme.dll to enable custom Windows XP msstyle themes has found a reg edit
in Windows XP Media Center 2005 to unlock the previously "greyed out" join
domain option. This method is pretty complex but confirmed to work.
This will allow you to leave and join domains within Windows Media Center
Edition 2005.
01 - Insert Windows XP CD.
02 - Start > Run > X:\i386\winnt32.exe /cmdcons
03 - (replace X: with the appropiate drive letter)
04 - Acknowledge prompt by clicking 'Yes'.
05 - Skip updating process by clicking 'Cancel' or by pressing 'Esc'
06 - After installation, acknowledge success msg. by clicking 'Ok'
07 - Start > Shutdown > Restart
08 - At boot menu, select 'Microsoft Windows Recovery Console'
09 - Select proper installation to log onto (default: 1) and press 'Enter'
10 - (If prompted, enter Administrator password and press 'Enter')
11 - Type (without quotes) and press enter: 'cd system32\config'
12 - Type (without quotes) and press enter: 'copy SYSTEM C:\'
13 - Type (without quotes) and press enter: exit
14 - At boot menu, select 'Windows XP Media Center Edition'
15 - Start > Run > Type: 'regedit' and press enter
16 - Click on 'HKEY_LOCAL_MACHINE'.
17 - File > Load Hive...
18 - Browse and select C:\SYSTEM
19 - Specify key name 'BANANA' and click OK
20 - Expand: HKEY_LOCAL_MACHINE > BANANA > WPA > MedCtrUpg
21 - On the right-hand side, double-click IsLegacyMCE value
22 - Change selected value to 1 and click Ok. (THAT IS NOT AN L!)
23 - Click on BANANA subkey (under HKEY_LOCAL_MACHINE).
24 - File > Unload Hive. Confirm.
25 - Start > Shutdown > Restart
26 - At boot menu, select 'Microsoft Windows Recovery Console'
27 - Select proper installation to log onto (default: 1) and press 'Enter'
28 - (If prompted, enter Administrator password and press 'Enter')
29 - Type (without quotes) and press enter: 'cd \'
30 - Type (without quotes) and press enter: 'copy SYSTEM
C:\WINDOWS\SYSTEM32\CONFIG\SYSTEM'
31 - (Yes, you want to overwrite - Go ahead and do it)
32 - Type (without quotes) and press enter: 'del SYSTEM'
33 - Type (without quotes) and press enter: 'exit'
34 - At boot menu, select 'Windows XP Media Center Edition'
35 - Join domain.
READ THE ENTIRE ARTICLE. There is a post dealing with your PC from Toshiba.