MSoffice progs let you spicify passwords for various levels of access, but from what i remeber their security is not that tight. if it's reallly sesitive stuff you want to protect (work/MOD/Bodey and Doyle type stuff, etc) the best bet might be some sorta encription sofware- never had anyting that important on my pc.
if someone can invent the security system, someone else can bust it. The aim is to make the cracking of the security more work that the info is worth.
Personally i copy the stuff that is for 'my eye's only' onto CD-RW's. that way it's not on the PC to start with..