one thing you could try doing is directly addressing the error and changing permissions on that one file. open up a terminal window and type (w/o the percent sign):
% sudo chmod 644 /tmp/501/TemporaryItems/com.apple.SoftwareUpdate
you'll be prompted for your password. if that doesn't do the trick, try it again, only use 755 instead of 644 (i.e. i assume com.apple.SoftwareUpdate is a file, but it may be a directory, and that needs the execution bits set, which is what the 755 does [among other things]).
report back on if it works. if the sudo command gets you an error, cut and paste both your command and the error message here for debugging.