I was recently upgrading a Mac from Jaguar to Panther, and in the process of using Software Update to bring it up to spec with the latest updates, I reached a point at which Software Update would "unexpectedly quit" before even beginning the download. Even some separately downloaded installers -- those that included a license agreement, I learned later -- would exhibit the same symptom, right after the administrator password was entered.
The solution is the on-line Support area on Apple's web site. It's in Article 2575, and it took me a while to find it. If, as I did, you're checking macosxhints first, hopefully this will save you some time.
[robg adds: As the article points out, the fix is to disable the "Times RO" font. Down near the bottom of the document, there's a one-liner that reads 'Some versions of stand-alone HP printer software install this font,' along with a link. The link, though, fails to load the proper page at HP. So I did a bit of digging, and found the page. It seems the font is installed by the driver software for the HP PSC 1300, 2400, 2500, and HP Officejet 5500 printers.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040502100350626