I don't think one day down time could affect a website search engine ranking so severely unless it happens frequently. Whilst not trying to fan Linux V Windows debate but Windows servers are inherently less reliable that Linux, I run a small hosting company we get so much agro from Windows server we have completely stopped hosting on Windows platform.
For rock solid reliability you may want to change the technology that powers that site to Linux powered technology and you will experience excellent up time from most hosting company.

I agree 100% with you Temi.
Windows servers seem to suffer more from both reliability problems and the hacker attacks than Linux based servers.

Also a 1 or 2 day outage will not kill your S/E rankings like it used to several years ago.