There are quite a few things that can cause this.
Check in device manager to see if all of your drivers are up to date.
Have there been any live updates from security software? Check to see that your anti-virus is up to date.
A conflict in anti-virus programmes.Have you uninstalled any of these but there may be elements left behind?
Have you checked the connection settings for the desktop? Make sure that the IP address is the same as for the wireless router.