Windows 7 and 8 as such do support MIDI, which indeed does not seem to go away any time soon. The problem your shop people seem to face is that they cannot find a suitable "driver" software that connects Win 7 with the particular sound card they want to sell you. Complete systems off the shelf virtually always support MIDI.

Good news: the problems you faced under XP with various MIDI programmes interfering with volume settings etc. will no longer trouble you.