Desktop-Reminder can be uninstalled as every other Windows program - using the Add/Remove programs Windows dialog or by executing DesktopReminderSetup.exe again.
In some rare cases the uninstall routine can refuse work telling that the installation package of Desktop-Reminder is missing.
Downloading and starting the fix-tool from the following page should help:
Fix problems that block programs from being installed or removed
Warning: There are some vendors wrapping the original DesktopReminderSetup.exe file in their malware/adware setups. Unfortunately there is no possibility to prevent this. The only way to be sure, you the the right installer is downloading it from the original manufacturer
download site.
The original DesktopReminderSetup.exe is about 30 MB large and it's signed by the following signer: Polenter - Pawel Idzikowski.