Thanks freshman777. You're one of the few dependable people here. I couldn't download any further than block 10000012 no matter what I tried and I couldn't get to the red alert that enables an upgrade.
I uninstalled and deleted everything. Then I downloaded the SHA512 flavour of 1.10.3 for my 64-bit machine. After unzipping to my desktop all that popped out was a NXT folder with lots of files inside. The installer didn't pop out.
I clicked on the nxt Executable Jar File inside the folder and nothing happened. The same goes for the nxt application and other executables. I copied the NXT Wallet Internet shortcut onto the desktop, clicked on it, and the 1.10.3 wallet opened up and started the blockchain download. It also recognised my NXT balances.
I don't think that the wallet shortcut solution is what is supposed to happen. I don't have the two NXT blue icons I had before for the NXT wallet and the NXT desktop server - just the single Firefox shortcut. But thanks for throwing me a lifeline. Appreciated!
I don't use the Windows version of the wallet so I can't comment on the icons it creates.
Generally Windows users should download the .exe link.
https://nxtforum.org/nrs-releases/nrs-v1-10-3/The Hash: SHA512 is the algo of the PGP message JeanLuc uses to sign releases, it's not related to the checksum of the file you download.
The checksum is in sha256 strings for .zip and .sh
For .exe there is no checksum, there is a different verification procedure as stated in "The exe, dmg, and sh packages must have a digital signature by "Stichting NXT"." which I haven't used, I don't run Windows.
TL;DR:
download the .exe file if you're a Windows user. Don't be afraid to uninstall/delete everything and start from scratch. There is no wallet.dat file with keys to lose, Nxt is a brain wallet. Uninstall/install as many times as you need to, as long as you remember the passphrase your coins are safe.