I'm using Avast5 AV Free on Win7 64 bit, FX 3.6, HD monitor and am having absolutely no problems viewing videos on Youtube. Have had no problems with Avast5 at all, in fact. I uninstalled 4.8 in SAFE MODE, rebooted and ran aswclear.exe, rebooted and installed Avast5 in SAFE MODE (to be sure Comodo HIPS didn't prevent a clean install). Rebooted twice, just because I always do after installing a new program. Only other realtime SW running in normal mode is Comodo FW with D+ enabled.
FWIW, I turned off the Windows Defender built into Win7 before installing Avast5. Figured two antispywares running concurrently would NOT be good.