With major OS upgrades come broken programs. When you make a bunch of changes to something like an operating system, you can't help but render certain things incompatible. Apple's just released a helpful list of programs that no longer work with the latest build of Snow Leopard. For your convenience, we've listed all of the known broken programs below.
Software restricted from installing/migrating:
1. Parallels Desktop, versions 2.5 and earlier.
2. VirusScan, version 8.6.
3. Norton Antivirus, version 11.0.
4. Internet Cleanup 5, version 5.0.4.
5. Application Enhancer, 2.0.1 and earlier.
7. Launch2net, 2.13.0.
8. iWOW plug-in for iTunes, 2.0
10. TonePort UX8 Driver, 4.1.0
11. ioHD Driver, 6.0.3.
12. Silicon Image Sil3132 Drivers, 220.127.116.11.
Software that won't open.
1. Parallels Desktop, 3.0.
2. VirusBarrier X4, 10.4.4 and earlier.
3. SPSS 17, 17.1
4. Director MX 2004, 10.2
5. EyeTV, 3.0.0 to 3.1.0
6. Ratatouille, 1.1
7. Aperture, 2.1.1 and earlier.
8. Keynote, 2.0.2 and earlier.
9. Airport Amin Utility for Graphite and Snow, 4.2.5.
All of these programs have been moved automatically by Snow Leopard to an inaccessible folder on your hard drive. If you attempt to use this software, you will be warned that it is incompatible and cannot be used. At that point, you'll need to visit the vendors website and download a patch to restore compatibility.
This is why it's usually best to do something like an OS update on a Friday. Get it done today, and spend the weekend fixing whatever it broke. By the time Monday rolls around again, you should be back to full functionality. It's not the most enjoyable way to spend a weekend, but it is prudent.