I've seen something similar. In my case, I'll attempt to start MC and while the process will start, no MC window ever shows up. If I kill the process and try again, MC appears as normal.
I'm not sure why it's doing it... it has only done so with build 312 (I haven't bothered to change to 314 yet, because none of the listed changes seemed remotely close to anything I do with MC). It's also not consistent... most of the time, MC starts up fine.
I don't have any media servers or PVR devices installed. I too only have one monitor. I'm not running any additional software that should have an affect (antivirus/antispyware/firewall/etc.), nor am I running anything that I haven't been running for the last 6 months (with no problems).
System info below... if it might help:
Media Center Registered 11.0.312 -- C:\Program Files\J River\Media Center 11\
Microsoft Windows XP 5.1 Service Pack 2 (Build 2600)
AMD Athlon 1416 MHz MMX / Memory: Total - 1048 MB, Free - 733 MB
Internet Explorer: 6.0.2900.2180 / ComCtl32.dll: 5.82 (xpsp_sp2_rtm.040803-2158) / Shlwapi.dll: 6.00.2900.2713 (xpsp_sp2_gdr.050702-1513) / Shell32.dll: 6.00.2900.2620 (xpsp_sp2_gdr.050225-1820) / wnaspi32.dll: N/A
Ripping / Drive D: Mode:Normal Type:Auto Speed:Max
Drive E: LITE-ON LTR-24102B Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: No / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive E: LITE-ON LTR-24102B Addr: 1:1:0 Speed:24 MaxSpeed:24 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: No
Use playback settings: No / Normalization: None