If all you do is internet, email an watch YouTube videos, a four year old Mac mini would last you the next 4 years. Your 4 year old PC would as well, as long As you don't load it up with a bunch of malware.
No, my windows system is 7 yrs old and I've max'ed the memory out at 4 GB yet I'm really only getting 3.5 with itunes and norton running alone it's gettin slower. 7 yrs it's time for a new one but mac mini or another dell. Can't decide.