You may try to Ctrl/Cmd+Alt/Option+Shift during startup or Move the folder with Illy closed.
If it is not something in the Adobe Illustrator CS4 Settings folder, it may be (with tests):
1. Corrupt font(s): you may move all fonts out of the Windows fonts folder (and move them back in);
2. Firewall interference (especially if more than one is running (XP SP2 adds one if activated)): you may turn them off (and on again);
3. Some versions of Acrobat generate many temp files. Too many (64K) causes the problem: you may search and destroy Temp files;
4. Some other inconsistency caused by Adobe Acrobat/Reader: you may try uninstalling and reinstalling (a newer version).
5. Incompatible/old printer definition file for the default printer: you may upgrade the driver, change the default, or set the PostScript driver/PDF to default;
6. Using a US/English version of Illustrator with Windows set to a different language: you may enter Control Panel>Regional and Language Setteings and change to English (and change back).
7. Quicktime/iTunes: uninstall Quicktime and make sure the Quicktime folder is deleted in the Program Files folder; and/or uninstall iTunes.
If none of these possible solutions work, have you made any (other) changes to the PC between succes and failure? New software loaded?
In the end, you may have to rebuild. In that case, remember to use the Cleanup: http://www.adobe.com/support/contact/cs4clean.html
Where is the CS4 update?
Next time will be required to learn programing to fix Adobe's failures.