Hi bnownlater,
First off, why didn't you update to 4.22? You know there's a huge security whole, right?
Anyway, on to the question.
You might be able to run your script in the background using exec(). I haven't been able to find the Windows equivalent of "&". But if you can, try this:
exec("C:\PHP\php.exe C:\path\to\script\to\execute 1> C:\errormessages.txt 2>&1 &");
Like I said, I can't find any Windows equivalent of "&", if anyone has, please let me know.
Edit: I'm an idiot, I thought you meant mass mailings. :o