Sometimes my pc get automatically turn off and

there is some hardware problem.Most chance is your ram is creating problem.clean the port of your ram may b there is dust.if still restarting then change the ram

There are a lot of items that can cause this from spy-ware (different to virus) to almost any hardware problem, to a simple overheating issue.

You need to work out which of those it is.

My first step being the easiest would be to take off the side of the computer after disconnecting it to check for dust build up in and around the cooling fan and power supply.
This is easily fixed with a can of compressed air meant for cleaning computers and just requires that you take it outside and blow it all out whilst wearing glasses and without poking the nozzle into anything.

Ram slot problems are quite rare nowadays but it may not harm to just remove it and blow out the slots and then put it back in.

Check that any extra cards are also seated fully as this can happen esp. if the computer was moved or leads were pulled that connect to them.

For Antispyware I generally recommend that using a clean known computer that a copy of MBAM is downloaded to a flash drive and then copied to the desktop. of the suspect computer.
Run it from there but do not change any options just accept the defaults.

when it is installed and has run let it delete anything it may have found.

After that I would run full and deep anti-virus scans with a good anti-virus (NOTE that Norton and Mcafee are NOT on this list).

some files may be missed..., restore your os ...