Nobody can tell you anything without proper system info or other technical details like the relevant comp settings, MP settings, what's going on in teh comp and so on.
If you really are doing a RAM Preview -- hitting zero on the numeric keypad, and not hitting the space bar -- your machine is committed to it. If you switch to a different application, AE will srtop generating the preview.
If you want shorter previews, shorten the AE Work Area.
Thank you both for answering my question. Yesterday the technical support from Adobe called and took control of the PC. He found that the 'RAM reserved for other applications' was set to high for the amount of memory the is installed on the PC (24 Gb). He brought it down from 16 Gb to 8 Gb and the problem disappeared.
To Mylenium, I know that I should have provided all the details about the PC and I forgot about it. I understand that it is hard to help troubleshoot a problem when you don't have the facts in hand. Sorry about that.