It isn’t compatible with all things Windows. That is sort of the point and the problem. ;-)
There are emulators that let it run a significant amount of Windows software (see Codeweavers for an idea of such stuff.) However, the idea is to use Linux native applications that substitute for what you find on Windows. Examples: Evolution, or Thunderbird as an email client, Firefox instead of IE, OpenOffice instead of Word/Excel/Powerpoint, etc.
I have two machines at home. One has Linux the other has XP. I boot the XP machine (a laptop) once every 2-3 weeks. The Linux box is net connected 24/7 behind a hardware firewall. Never has problems.
That's good to know. Thanks for all the great info. DH is using Firefox and so far he's had no problems with Ubuntu. I think I'm the worrywort! LOL