I have windows 7. I know C# and web services (HTML, CSS, JS, ASPX). I want to develop mobile apps with my skills for iOS (the most important option) and android. I'm looking for a studio platform that I could develop there with my skills, something that I can build with my own knowledge and would automatically be adjusted to all platforms. I looked at Xamarin but I can't do it without Mac. PhoneGap doesn't work. Which platforms could you guys recommend me?

Any more explanation beyond 'PhoneGap doesn't work'? I ask because PhoneGap does work, proven and tested by a lot of developers, myself included.
I would have suggested either of those two you list because there aren't many other options. A mac VM is apparently an option but the last time I looked into that it was a breach of Apple's restrictions and therefore technically illegal.

If you want to develop iOS or OS X applications, you have to have a Mac to do the final packaging on (either you or an agent of yours). If you use our "8th" development language (8th-dev.com), you can do all your dev work on Windows (or Mac or Linux), but you still will need a Mac to do the final signing etc.