mine will usually sit at around 30-38% with just the start up programs running and shoot up to about 42-45% with IE and wmp running (i have 2 gig ram) it maybe that norton or another program has run an update which has installed additional programs/drivers to your system.

it could also be that you may of picked up a few unwanted programs (adware etc) so i'd do a full system scan to be safe

if your using norton, just make sure your not paying for it