CID remains in background

I’ve installed the portable versione of IceDragon on my WinXP sp3 (updated). I noticed CID remains in background after I close the program, i. e. the icedragon.exe process is present in task manager list. In this way I can’t reopen CID because it’s already in execution.
The same happens with the installer version of CID.
No problem with Comodo Dragon (CD).
Any ideas to solve this issue?