I have a concept on how you could get it to work, but I'm too lazy to try it.... so I'll just explain my idea.
1.) Create a background worker control just to keep things simple.
2.) Add the image you wish to use as the slider, and instantiate both a MouseDown, and MouseUp event for it.
3.) Create a global variable called IsSliding or something like that.
4.) In the backgroundWorker_doWork function, make a while loop that continues looping while IsSliding is true.
5.) When the MouseDown event occurs, set IsSliding = true, then call backgroundWorker.RunWorkerAsync(), for MouseUp set IsSliding = false
6.) In the backgroundWorker_doWork function, continuously grab the X position of the mouse on your screen, calculate the relative coordinate of the mouse on your form based on the x/y of your application, and set the pictures X coordinate to that coordinate.
Then the only thing left to do is to make sure the picture isn't able to be moved passed a certain X coordinate in either direction, there are a lot of ways you could accomplish this but I'll leave that to you to figure out.
Hopefully this helps
Edit: You might also consider adding some type of handling in the backgroundWorker_doWork function to make sure that the application is in focus; then if it goes into the background end the doWork function.
Also, in order to adjust the seek accordingly to the correct position what I'd do is calculate a percentage based on the max/min position of the slider and its current position, then calculate that percentage into a time based on the total length of the song/video.