In deference to what hoodoorocket has stated, CrossOver appears to have quite a few advantages over Parallels indeed.
But, to answer your question first. It is highly unlikely that your Mac could be infected by a virus running on CrossOver. This is because each application that you have installed is on its own virtual machine, or "bottle." If an application is infected, simply delete the bottle, create a new one, and move on. The virus would also not likely escape to infect the rest of your mac, and if it did, would not get beyond your own user account. But, this is highly unlikely to begin with. They explain it quite well on their website: http://www.codeweavers.com/support/forums/general/?t=27;forumcurPos=1600;msg=43695
Now, as to the benefits of CrossOver to Parallels. There appear to be many. If you get a virus under Parallels, your entire environment is compromised, and you may have to reinstall Windows and risk all your files on that system being compromised as well. This, as stated above, is just not possible with CrossOver.
Parallels is FAR SLOWER than CrossOver. I could not even get Galactic Civilizations II to run under Parallels. It works like a charm, and very fast under CrossOver.
CrossOver runs apps from multiple Windows platforms (XP, Windows 7, etc., even legacy e.g., Windows 95)... you would have to have multiple installs under Parallels to match that...
There is no rebooting under CrossOver... of anything. You simply close the program.
You don't have to purchase Windows to run Windows apps by using CrossOver. Period. Hoodoorocket should have investigated his suspicion before sewing it. I have investigated it, and can attest to the veracity of CrossOver's claims.
Bottom line, CrossOver is far simpler. An app is behaving badly? Delete it's "bottle" (a folder in the CrossOver folder created when you installed it) and simply reinstall the file. Try that with Parallels... I think you will find it to be hardly simple at all.
On the other hand, there are some glitches... just as you would have with Parallels. I have found some software that CrossOver does not run... but, again, there are quite a few that Parallels doesn't either, and that it runs quite slowly. Such as GalCiv II.