My pc is freezing randomly. My desktop is a i5 2500k build

The basics for a PC to boot is CPU + MB + RAM. So first test these to make sure it is not them.

Hard Disk not detected. Make sure the drive itself is ok and the port on the MB is ok as well as the SATA cable.
Make sure the front panel cables are not close to the SATA cable, this has caused a problem before.

Check that your drivers are all working ok and reinstall them if it is necessary.

If this does not work make sure that none of the components are running too hot. Install speedfan to check this, it work pretty good.

I am often getting a "nv4_disp"(Nvidia graphics card error) bluescreen. Can this be the cause behind the random freezes? Sometimes it freezes for 15-30 seconds and sometimes it never unfreezes.

Then I would suggest looking at the Graphics card drivers and re-install them. Do a stress test on the card and see how quick your system freezes. See if there is an update for the bios of your card and update if necessary.It could also be the slot on your motherboard that could be the fault. Remove the card and see if that helps and swop it out with another card to check if it freezes. If it doesn't then you know it is the card. Contact your vendor where you bought the card and send it in for testing as there could be a fault with the card itself.