In all likelihood you're using some kind of security software or 'cookie cleaner' that's deleting your logon cookies as part of a system scan. They can be deleted manually Tools/Options/General/Browsing History/Delete/Delete Cookies, and that's what I'm guessing is happening.
What security software and/or scheduled tasks are you using at the moment?
About once a month I have to reinstall my “user name or I/D’s” to log on to various sites. Is there a way to keep them? I am using I/N Explorer 7.
I know the latest versions of Zone Alarm have a cache cleaner built in, and I believe they're set by default to scan and wipe every few weeks, so it's probably that. Look in the Zone Alarm control panel, and disable the cache cleaner in the Privacy tab.
I wasn't under the impression IE7 deletes anything unless you tell it to, but some websites will automatically expire logons after a period of days or weeks. I believe eBay is like this.