Secure Cookie-based Login

I have researched several login scripts and I have a few questions that I haven't been able to find the answers for.

First if of all, my goal is to create a cookie-based, non-SSL, login system. I have many, many users that are going to be logged in for extremely long periods of time, so I absolutely do not wish to use session variables under any circumstances.

I've come across several great algorithms and one-way hash's that seem to work great for encoding the password. However, they all seem to have one thing in common. Once the user successfully "logs in", the site simply sets a cookie using the User ID of the login account. On all password protected pages, the only check that is made is the User ID stored in the cookie- compared to the value in the database.

I'm curious if it is possible for a hacker to create a false cookie on their system, storing simply the UID. Is it possible to do this, or is there some kind of internal OS security that prevents such an action? In other words, if I go to a co-workers computer, review their temp files, find the cookie for the site they're logged into and take the UID... Could I recreate that cookie on my own computer at a later time and gain entry?

This may not be so much of a security issue on a site that only stores a temporary cookie, but what if when they login, I set the cookie to expire after a year? (so they don't have to login every time they visit the site).

I'm not saying this method isn't secure, I guess I'm looking for an explanation. Also, any ideas on where I can obtain more information about creating a login application that's not based on session variables?