As a test, turn off your router but leave DSL modem on. See if your desktop gets much faster. If so, could be that nearby neighbors are stealing your wifi.
With router back on, go into wifi settings and view which devices are linked.
Type 192.168.1.1 in a browser.
Log in screen will ask for user name and password.
If 'admin' and 'admin' work, then you have no router security and are operating an open wifi network. If those don't work, you may have set them a long time ago. Try some name/password combinations. A super bad neighbor would not only 'borrow' internet access but could reset your router so only they have control over it. One thing you do have is the reset button. Press in with a toothpick or pen tip and start over. Select a name and password, a name for your network. Select WPA or WPA2 type security. Use a long security key.
You can choose not to broadcast the network name also. Save settings and restart.
Also scan your computers for malware. From time to time I run a full scan of my Windows Op system using Microsoft's Standalone System Sweeper. It is basically a boot CD disk that you burn with standard MS detection tools and root kit removal ability. Belief today is that if you are running these types of scans from within the Op System, you are wasting your time. The malware has already told your detection software to report 'everything ok'. Only a boot disk stands a chance. http://connect.microsoft.com/systemsweeper
It is basically MS Security Essentials that Wildie suggests above, but run from a bootup disk to avoid malware in your boot sector.