The question is, why do you want to ignore the low disk space warning? It's letting you know that you have very little space on your hard drive, which means either a) it's time to add another drive, b) it's time to audit your installed programs & files to see what you really need installed or saved on the drive vs. what can be archived onto CDs or DVDs (i.e. old photos you haven't looked at in 5+ years), or c) you need to see if there's a reason why the space is filled up (i.e. too many old temp files from app installation) that could be cleaned up.
As was pointed out, just wanting to edit the registry to prevent the warning is a very bad idea, especially if you haven't investigated why it's popping up.