Billboards are very expensive; spamming message boards is free.
If you see a spam account that hasn't been dealt with, feel free to report it to a Mod/Admin.
Sounds like you have an existing paint.net installation that has become broken. Please use this utility to remove the broken installation:
https://support.microsoft.com/en-us/help/17588/fix-problems-that-block-programs-from-being-installed-or-removed
Afterwards, you should be able to install paint.net anew.