Try the following:
start > run > type: regsvr32 softpub.dll <enter>
start > run > type: regsvr32 initpki.dll <enter>
start > run > type: regsvr32 mssip32.dll <enter>
Still can't login after that? Look at the first two posts in:
Possible solutions for "We were unable to sign you in to .NET Messenger"