I haven't ever used it, but if it's happening when you turn the computer on, have you checked the list of your start up programs?
I'm no computer expert but just an educated guess to me it sounds like there's some part of it still there that's launching at start up, tries to access it and is mad that it's gone. I could be wrong, but wouldn't hurt to take a look.
I asssume you deleted it through control panel - Add/remove programs.
Also its probably not a virus, but just some files still stuck on your computer.
You can check on your start menu (Start-All Programs-Startup) to see if there is any trace left of it there.
You can also type "msconfig" into your start menu's run/search bar, and check throught the different panes to see if there is anything remaining of Curse client there.
If that gives no result, just check your program files to see if the Cruse folder and files really is deleted (only delete them if you already uninstalled the game the correct way, if you do this without going through Add/Remove programs or an uninstaller wizard that might cause the problem you are having)
Type MSconfig on the Search button
go to startup in MSconfig app
Look for Curse client Startup tab
Go to Boot Tab
Make All boot settings permanent
Go to Task manager, Find the process (if its still running) Right click and open File location.
End the process, then delete all the files in the folder.
if its not in the task manager then browse to the following location might take a bit of time, and you will need to un hide the folders (plenty of information on that elsewhere on the net)
C:\Users\(Username)\AppData\Local\Apps\2.0\Random Characters\Random Characters\
then delete all the folders with curs (More Random Characters)
Grab a copy of CCleaner, and use the registry scan for issues, and fix any that come up.
Reboot. That should stop it starting up.
The Actual install of the Curse Client is one of the nastiest installations I have seen, it is very much like a virus in that it hides its installation very well inside the user folders, rather than the normal installation locations.