I was looking to theme my PC in an XP style but nowhere can I find a reliable way to replace my icons (i.e. Recycle Bin, Hard Drive, CD icons) with XP's. I was curious as to whether I could perhaps create an XP VM, pull the shell32 off it and replace my W7's. Alternatively, are there other avenues I could try?
No, you can't do that.
Shell32.dll is a core file of the operating system which provides the functionality needed to make the shell works (although it's a simplification the shell is more or less Desktop + Start menu + Windows Explorer). Yes, it has a lot of icons but only because that icons are used by the shell so they are stored the same place.
So changing that file would affect a very important part of Windows that need to be synchronized with the rest of files of the OS, even using another file from the same version of Windows but with a different Service Pack applied would make the operating system unbootable.
Anyway, nowadays you couldn't even try that because starting with Windows XP (or maybe 2000) Microsoft added Windows File Protection, which maintain a copy of the core operating system files and if it detects that one of those files have been replaced it replaces it again from the backup copy.