Uninstall your sound drivers and hardware in Device manager. Reboot the system and then refresh Device manager if Windows does not find your hardware automatically.
For downloading try these threads
http://www.vistax64.com/vista-general/6555-downloaded-files-disappear-actually-never-appear.html
http://www.eggheadcafe.com/software/aspnet/29384013/uac--protected-mode-prev.aspx
http://www.vistax64.com/vista-security/43692-uac-protected-mode-prevents-any-downloads-ie7-2.html
Could also be the problrm with nuAC blocking programs at startup, Sound driver or program??
Quads
I have been thinking, first time round the restarts went to a startup repair utility of sorts that you stated, that could have during the "repair" meant that the likes of the UAC settings are back to like a new PC,
Quads