Fuseblown

I have a problem with fuse 2 on the motherboard. Everytime i switch the standby on the fuse blew up. I took a look inside and found out that the 250V/500mA slow blow fuse 2 was broken. I used my multimeter to mesure see how much Volts there are and its around 300. My question is: does anybody had the same problem and knows how to fix it? I dont want to pay hundreds of bucks to fix it if its easy to do it on my own.