I have reconnected my keyboard and mouse to the iMac I just got out of the shop. I can double click and open files, but I can't drag them (like to the trash). I tried another mouse, but still had the same problem

You may not be double clicking fast enough. In the Windows System Control Panel, there is a Mouse Settings control panel. You can adjust the double click rate in the Mouse Settings control panel.

I’m happy to help further over the phone at https://www.6ya.com/expert/jeremy_69f3cc28d95bf514

To modify the mouse settings, open the property sheet using Start > Control Panel > Printers and Other Hardware > Mouse and select the Buttons tab as shown below. Check boxes and sliders allow the necessary modifications to be performed.
There is even a Click Lock function that is very handy for those that have trouble holding down the mouse button while using the drag and drop function.
The button configuration and Double-click speed settings must be set properly. Then it can solve your double-click problem.

Download your program. Make sure it is for YOUR OS. A .dmg should appear on your desktop when it is finished. Double click that. It should open a file with an application icon and a drive icon on the desktop. Open your applications folder from your hard drive, then drag the application icon to that. Eject the drive icon from the desktop by dragging it to the trash.

Check for radio interference with phones, microwave oven, etc. (unplug them for the test).
Yoour PC may also have malware, causing the cursor to jump (but that does not usually cause the double-click problem)

I had the exact same problem with my original Logitech wireless mouse that came with a wireless keyboard. I eventually figured that the button component inside the mouse wasn't making consistent contact causing unintended double clicking or dropping while dragging. I fixed it by doing some soldering to switch the left mouse button with the right mouse button, since I didn't have to use double clicking or dragging with the right mouse button. It worked pretty well for several months until the switched left button started showing the same problem again. Now my newer Logitech mouse (it's more expensive Nano wireless mouse) is having the same problem after about 1 year using it. I think there's a serious durability problem with the button component that Logitech is putting on their mice. I may do the same trick again on this mouse, but now I don't consider Logitech as a good mouse manufacturer anymore. They used to make good quality hardwares but I guess they aren't that good anymore with all those cheap Chinese suppliers for their parts. :(

I suspect you have a wireless mouse. When the batteries get low in an Apple wireless mouse, one of the prime symptoms is that click works but drag doesn't, or vice versa. Replace your batteries.

Check the CCORNELL posting for the solution. Your mouse is double clicking when it should be only single clicking.

try to set on your MOUSE, here:

start-control-panel-mouse folder

hope this can help. :)

Plase try to lower the double click speed which is found in control panel-mouse-pointer options Hope this will slove your problem

in most cases the best thing to do is to change ur mouse, but i would also suggest u try to use a mouse pad, some paints have a problem when using optical mouse