Well my old PC I always had to because I wasn't really educated and I went to websites that gave spyware and stuff and I didn't know what to do and it slowed my PC and "pop-ups" came up with naughty stuff soo had to always reformat
This laptop is a year old and I havnt reformatted once. I have four programs running in the background to catch any viruses but have yet to get any serious ones.
I formatted my PC a couple of times but haven't got any problem for the last one year. My advice is stay away from porn related sites and your system will be hale and healthy. If at all you venture, go there well protected with a good antivirus