In Windows Vista (and Windows 7), the ‘Temporary Internet Files’ folder is located at:

C:\Users\Owner\AppData\Local\Microsoft\Windows\Temporary Internet Files
Assuming that your Windows is installed on the C drive. To see it, not only do you have to check ’show hidden files & folders’ option in the Folder Options, but also uncheck ‘Hide protected operating system files/folders’ option.

But wait a sec, where is the index.dat file ?

To access it, you will have to type in Explorers address bar the following location and click go:

C:\Users\Owner\AppData\Local\Microsoft\Windows\Temporary Internet Files\Content.IE5

Only then will you be able to see the index.dat file. Conclusion ?
The Content.IE5 folder is ’super’ hidden!