Make sure your version of MC is up to date. Copy your system info from MC Help and paste it here.
Then, in MC, open Video in the tree, then Web Media, then Youtube. Do they play there? That's how it should work.
Media Center 12.0.279 -- C:\Program Files\J River\Media Center 12\
Microsoft Windows XP Workstation 5.1 Service Pack 2 (Build 2600)
Intel Pentium M 1695 MHz MMX / Memory: Total - 523 MB, Free - 193 MB
Internet Explorer: 7.0.5730.11 / ComCtl32.dll: 5.82.2900 / Shlwapi.dll: 6.0.2900 / Shell32.dll: 6.0.2900 / wnaspi32.dll: N/A
Ripping / Drive E: Mode:Normal Type:Auto Speed:Max
Digital playback: Yes / Use YADB: Yes / Get cover art: Yes / Calc replay gain: Yes / Copy volume: 32767
Eject after ripping: Yes / Play sound after ripping: No
Burning / Drive E: MATSHITA UJDA755 DVD/CDRW Addr: 1:0:0 Speed:8 MaxSpeed:8 BurnProof:Yes
Test mode: No / Eject after writing: Yes / Direct decoding: Yes / Write CD-Text: Yes
Use playback settings: No /
Portable Device Info
Removed devices:
Interface Plugins:
Library Server (Active)
TiVo Server (Active)
UPnP Server (Active)
Album View 2.0 (Active)
Auto Import (Active)
Chart Finder (Active)
Cover Art Finder (Active)
Playing Now (Active)
Sleep Timer (Active)
The Master (Active)
I can play the videos that I have not downloaded that are still on the youtube site, but the videos .flv fiels that I have saved cannot be played. My understanding is you should be able to save and play the .flv files offline. Is that not the case?