Safety first!

Is my computer at work really housing such ultra-sensitive data, and so suceptible to attack, that it is necessary for me to change my password every month? It's enough of a challenge already for me to remember the rotating handful of passwords I keep in mind for my couple of email addresses, online banking, etc. And now here comes along this totally over-anxious requirement that I not only think up a new password every month -- which, by the way, can't be any password I have ever used before (!) -- but also, it must contain at least one capital letter and one number. Seriously? Who, exactly, is trying to access my pitifully formatted Excel spreadsheets that such measures could be useful? IT types, I'm all ears on this one.