stuck on a Linux server?!? Earth calling Andrew!
Performance, security, stability.
So I wouldn't even consider hosting anything mission-critical on a Windows machine. In an ideal world your server would show an uptime of a couple of years, and I've only ever seen that with Linux or BSD boxes. A single linux machine will also cope painlessly with that many visits per day, I wouldnt feel confident of that with a Windows box (to put it politely!)
PHP is an straightforward choice (especially PHP5), and possibly MySQL though I might consider using a proper transactional database (I've heard Sybase touted around a lot recently, though I know very little about it) for something as big as an e-commerce site - you at least want to be able to perform rollbacks when things (inevitably) go wrong.
Since this is e-commerce, you want to make some money and so you'll probably choose to avoid the kind of licence fees and "upgrade" lock-ins associated with the Microsoft platform. You may even have a team of developers and want to implement proper source control, and so the single-user Windows platform is not going to work for you.
I love Perl but I would rather be unemployed than implement another e-commerce app using it, even if this were mod_perl. Certainly any CGI solution is going to suck big time once you get more than 3 visitors per hour - it just does not scale well, that's why people don't use it.
Anyway, enough rambling - I think I would go Linux (dedicated server down at Rackspace ), Apache, Postgres and PHP5. Hmm yeah, when do I start?