Well, without knowing which icon pack it was that you installed there are two different ways you can go about it.



First, being as most icon packs here are offred in .7z format you could have used 7tsp to install it, in which event you would use the 7tsp "Restore" function to uninstall back to the systems original icons.



Second, if you did not use 7tsp to install the icon pack, or even if you did for that matter, open an elevated command prompt and type: sfc /scannow (with the space) and hit enter, it will take a while to scan, after it gets to 100% it will tell you that it found errors and fixed them and they will be fixed on reboot. type: exit to close the command prompt and reboot, your default icons should be back to normal.



BTW, all an elevated command prompt is right click on command prompt and selecting "Run As Administrator"