The OS of choice for the general PCs currently is coming from Microsoft. As of this moment, the latest offering from Redmond is Windows 7. There are several flavours to choose from, depending on one’s requirement. Each of these flavours come in two version i.e. 32 bits and 64 bits.
I would surely recommend the 64 bits version. Its the future and most current hardware should be able to run it. More data is being moved and compute in a single clock as compared to 32 bits, it should be faster. Furthermore, currently it has much much less, practically none known viruses as compared to it’s 32 bits counterpart.
Linux provides a free alternative OS. If you have not heard of any, try searching for ‘Ubuntu’. It’s quite easy to use now as compared to few years ago. My personal belief is that it is still a lil bit geeky but otherwise easy. Hopefully there will be great improvement in the near future. I am waiting for that jump.