You don't need to physically pull out any RAM to make Painter X work on a 64bit system with a lot of RAM. Go HERE and apply this fix (it's a third party solution, so done at your own risk - tho I have done it with success). However this will have no affect on Painter 12 as the RAM limit barrier was fixed in this version.
Could possibly be that your ownership rights have become corrupted. Worse case senario is that creating a new user wont fix it. I had this problem. Although my Win7 Ultimate 64bit system was saying I had the 'right' to install a program or run it, it just wasn't working. Creating a new user didn't help. Ultimately I got rid of the problem as I reinstalled the whole OS (due to another problem). I had found I could install most things, but programs like Corel Draw refused to run untill I'd changed some user rights.
You could try turning off the UAC "User Account Control" settings (type in UAC in the search bar in the start menu to find it). You'll have to reboot, and see if that helps get the program running. It may not, so you can turn it back on again. If it does help then it would indicate that your admin rights have corrupted in some way.
Do this at your own risk, but you could try changing the ownership/admin rights to Painter, by going to the program folder, finding Corel Painter 12 folder and right click on it, select properties, then the sercurity tab, then the advanced button. Select Owner, then change the owner to your user name, tick the 'replace owner on subcontainers and objects' then ok. Then select the Permissions tab and click change permissions and give yourself full rights and tick 'replace all child object permissions with inheritable permissions from this object'. If your user name isn't on the list click Add, then Advanced, then Find Now. Then from the list add yourself. You can can change back ownership to Admin once your done.
There's also a file under ProgramData/Corel/Painter12 called PF12.dta that may need the same thing done to it. Again do it at your own risk. Though as long as you only touch the Coral Painter folder (since it's not working anyway) you're probably not risking much.
I only have one other thought. Make sure you have the Protexis Licensing service running (go to Task Manager / Services). On a 64bit system there should be two, PSI_SVC_2 and PSi_SVC_2_x64. This is Painters licence program to validate the serial which runs in the background. Painter wont work if they aren't enabled to run.
You could also check if your Mircosoft.NET Framework NGEN v4 (should be two of them there) have installed properly - Painter installs them. Click the services button at the bottom of the Services tab, and from the full Services list check all these are set to automatic. If you have the v2 of the NET framework installed these I think should be Disabled (at least mine are).