You would have installed it, things like norton, kaspersky, mcafee etc?
The quickest way to test for software issues is to load windows in safe mode with networking (press "f8" during start-up", and see if it's still slow.
It's possible there could be a wireless issue.
You may wish to try connecting the PC to the router via ethernet cable to see if this eliminates the issue. If it does then we know we can focus on the wifi.
If yu are unable to try this you may wish to try logging on to your router management web page and try changing the wireless channel.