Computer shut down suddenly when usb flash drive is plugged in. What is the problem?

Virus issue . i faced this issue after that install the free ware microsoft security essential AV . after that issue get resolved ... 

Looks like USB Drive is infected and have virus which shutdown PC or USB Have hardware problem.

It is highly likely that your usb port had a power surge that cause pc to shutdown. Or otherwise that Flash drive had a virus.

Dear may be you flesh drive is coppupted by virus or damagded 

Regarding this problem there are cretin issue to look in to, if PC shuts down as the moment USB is plugged in then USB port is having power surge(short circuit) or USB device being plugged in is having power surge(short circuit). If computer shuts down after few seconds or couple of minute after plugging USB  then there is a serious virus problem, get a nice antivirus like SMADAV or MS Security Essential and scan the USB as you plug it in.

Computer virus is the suspect based on my experience testing it both on Mac and Windows PC.

I had a client that brought a flash drive and warned me that it shuts down computers. Despite knowing it, I tried to plug it in and after few seconds the windows PC shut down. So I restarted in and when it opened, a virus was detected. So what I did is pull-up CMD and did manual removal of viruses. By the way, I never opened the folder. I used the del .lnk and attrib commands to do this. Which resolved the issue.

The next day, my client came back and told me the same thing happened, so I tried it with Mac PC and yeah it shut it down but this time after like 5 mins. So I did a deep scan to analyze the virus name and it was detected by AVG and it was sort of a trojan virus and malware but I forgot the name. 

So how was the flash drive fixed? As usual, I used manual virus removal and advised my client to format his computer. He came back to me and he said it was his computer that breeds computer virus causing his flash drive to carry auto shut down commands.