Install this program, then click the update button to get the latest definitions, then run it. It'll run for a while and stop and offer you the chance to mark a check box of different things it found. If they're not already checked, right click on one and choose select all. Hit continue until it's finished and you're done.
You can also check http://windowsupdate.microsoft.com for any new operating system updates or security fixes.
Also http://officeupdate.microsoft.com has the updates for microsoft office.
One last thing. Make sure you have the latest antivirus definitions. I use mcafee. Their definitions can be found at ftp://ftp.nai.com/pub/antivirus/datfiles/4.x.