You need anti virus software on your Windows partition. Download and use Microsoft Security Essentials. It is the only anti virus program you need. I do not recommend bloated programs like Norton, etc.
2ed that bloated virus scanner programs only make money because they com preinstalled on most computer bought with windows and most users don't know any better then paying up when the trail time is over
As Bob has noted, you need anti-virus software on the Windows system. Anti-virus software on the Mac will not protect the Windows system, unless you cut that Windows system off from the internet entirely and scan all files on the Mac side, with a good anti-virus that does well with Windows malware, before transferring them to the Windows partition.
If you're wondering if the Windows system will somehow risk infecting your Mac OS X system, and whether you need anti-virus on the Mac, there are two issues to address. First, the Windows system isn't going to infect your Mac system, so that's not a concern at all. Mac OS X is not more or less susceptible to infection when there's a Windows system on another partition.
That leaves the question of whether Mac OS X needs anti-virus software on its own merit, regardless of the presence of a Windows system. That question is answered in my Mac Malware Guide.