Ok good to know everything is ok - however I think your problem might be that Windows 8.1 may simply be a very demanding piece of software to run on your machine...
Your best bet might be to upgrade your HD to an SSD - but check your motherboard supports at least SATA II (3Gb/s) first, otherwise I think you might do better to start looking at a new system...
If you do go the SSD route, bare in mind that if you then decide to upgrade anyway, you'll be able to carry the new SSD into the new system, so it's never going to be money wasted