Laptop troubling a lot!!!

Tech Support Guy System Info Utility version 1.0.0.2
OS Version: Microsoft Windows 8 Single Language, 64 bit
Processor: Intel(R) Core(TM) i3-3120M CPU @ 2.50GHz, Intel64 Family 6 Model 58 Stepping 9
Processor Count: 4
RAM: 3961 Mb
Graphics Card: NVIDIA GeForce GT 720M, -2048 Mb
Hard Drives: C: Total - 435734 MB, Free - 358283 MB; D: Total - 25599 MB, Free - 20052 MB;
Motherboard: LENOVO, INVALID
Antivirus: Avira Desktop, Updated and Enabled

Nowadays my laptop takes a lots of time(15-20 mins) to start and sometimes it doesn't even starts. After starting, it works properly. Can't understand whats the problem...
please help!!!