When configuring MSE to run a schedules scan, there is a check-box below which says, "Check for the latest virus and spyware definitions before running a scheduled scan"
On my 64-bit PC with 4 gigs of RAM, a "quick scan" takes about three to five minutes and runs completely in the background - allowing me to proceed with whatever I'm doing at the time. If you check that box, MSE will search, download and install the latest definitions before running the scan -- which is how I receive my virus definitions updates daily.
I also run a full scan every few weeks - which obviously takes much more time; however I have made the MSE quick scan part of my daily PC protection scheme. You can also choose other options - such as to run the quick scan weekly or monthly.
Give it a try. If you find that it's not for you, you can always change the configurations back to what they are now.