Solved it! After trying to re-install Seti@home and getting the same result, I realized (I suspected it anyway) that Abacus wasn't to blame. A search on the web turned up the solution.
In the Windows folder there is a subfolder called 'Repair.' I simply copied the file in question (Autoexec.NT) to the System32 folder and it's as good as gold!
Amazing what you can discover when you google something...
