Let's say that I want to delete index.dat from the command prompt,
what's the difference between del index.dat/s and del index.dat /s?

Thanks.

The command may assume that index.dat/s is the name of the file you're trying to delete - ie, a file called s in a folder called index.dat, whereas with the second one its specifying a command line switch to silently delete the file (which is what you want to do).