When playing .mkv files and .mp3 files in both windows vista and windows 7 my system will unpredictably freeze. The mouse will still move and the start button remains interactive but i cannot navigate through or open files. When the system freezes the HDD light becomes solid. I replaced the RAM, but the problem remains unchanged. Does this sound like a HDD problem?
I would normally say that it seems like a Windows issue, but if it is doing it on two separate OS's, it is surely hardware related. What sort of graphics card are you running? PSU? To ensure the HDD isn't the problem though, try running an HDD utility and see if it throws any SMART errors.
It might be worth trying hdtach, that'll give you a readout of speed across the drive and cpu usage, if speeds are all over the place and cpu usage is over the top it might hint at a problem.
I wouldn't necessarily class Vista and Windows 7 as different OS on the driver front either, if the chipset manufacturer did iffy Vista drivers they are probably the same ones on 7 too (though I assume Microsoft can still install some default drivers). I deleted my XP and installed 7 on my main machine and had some stalling problems and media player was very ropey. I reinstalled in 32bit and it seemed better, but still prone to odd lockups etc. I had various bits of software that it didn't like and so went back to XP.
Might be worth saying what motherboard/chipset you have.