I don't know how secure it is, but in Vista, IE7 has the Protected Mode, and also asks when anything tries to open a document in an external app (unlike IE7 on XP), for example, Acrobat... Also, while it looks out of place in XP, it actually fits nicely in Vista, whereas Firefox (with the default skin) looks nasty.
IE7 starts faster, and I have yet to notice any dodgy stuff installing itself. But then again, I don't blindly visit random sites, and don't run the sorts of attachments like "Hot_Women.ppt.exe". In fact, I think my antivirus quarantines files with double extensions except things like .tar.gz .
Oh, also, in Vista 64, there are two version of IE. The 32-bit and 64-bit versions. Default is 32-bit so plugins work correctly. That's why I use the 64-bit one
No flash for starters. It's not really hard to open a copy of 32-bit IE for youtube links.