When you say "windows office", do you mean you're running Windows either via Boot Camp or one of the virtualization systems such as Parallels Desktop or VMWare Fusion? If you are running Windows in any form, you absolutely need antivirus and antispyware protection for Windows, just as if you were running any other Windows system.
If, however, you mean that you're running the Mac version of Microsoft Office, then the need is less pressing. There are macro viruses that can infect Office documents on a Mac, but if you have Office updated to the latest and you don't allow it to run macros unless you're confident of the source, you should be fine.
As to other exploits, at this time there have been no confirmed Mac OS X viruses (subject to the semantic debate about whether the iChat exploit is properly classified as a virus or a trojan horse) and very few trojans. I therefore do not feel that antivirus software is necessary at this time, but it's never wise to become complacent, so security precautions are not wasted. In addition, it's possible for someone to inadvertently forward a Windows virus to a PC user (though said Windows virus cannot infect the Mac).
If you do decide you wish to run antivirus software and you want software that scans and updates automatically, I'm not sure what to suggest. I used to suggest Symantec's Norton Antivirus, but enough people have reported problems with it that I no longer feel comfortable recommending it. But I think that the donationware
clamXav should be more than enough provided you remember to scan your system from time to time.
