H_R, that sounds to me like you have a hardware problem, start by running CHKDSK on your hard drive to see that you don't have any bad sectors that are affecting your Windows install. If all is okay I'd look at replacing your RAM or, if you have 2 cards of RAM, try taking one of them out in turn to see if that stabilizes your PC, or get them checked out at your local PC shop. I take it that you are running a competent anti-virus (at least)?
G_Man, as oxfordSteve has said, yours sounds like a driver issue, MS has a habit of updating drivers with the wrong one, find out what wi-fi driver you need from your laptop manufacturer's website and download the current version, uninstall the one you have using Device Manager and reinstall the correct one.