start from the software

In general, computer can last at least 1 to 2 years with any problem on daily use.(I means switch it off at night and run 8 hours a day)
The first thing I would try is on the software side. Disablee all the start up programs by "msconfig".
Then run Antivirus and Anti Spyware programs like AVG, Kasper, Hijackthis, Spybot, Ad-aware etc.
If the problem persist, then you might need to back up and re-install. The last concern is on the hardware side like memory, power supply, cooling fan etc...