The operating system in a used laptop will provide the user with all of the original functionality which it initially offered, even after the support for the specific version of the operating system has ended. The actual lifespan of the OS (operating system) will depend on which one your used laptop actually has installed on it.
Take Microsoft Windows XP for example. This is an extremely popular OS and is found on a variety of used laptops under 100 dollars. However XP has been around for a long time now, initially released in 2001. Microsoft has a specified support life cycle which addresses each of their operating systems. For Windows XP the original support ended in April of 2009 however this does not mean that any computer running XP becomes unusable by any means. The actual usability of the computer remains the same as it would have initially yet the updates, hot fixes and Service Packs which had been released under the initial support wind down. This has been the case for the Microsoft operating systems of the past and similarly to newer ones as well.
Windows XP in its most common versions includes Home, Professional and Media Center Edition. The highest Service Pack was SP3. As for the cheap laptops which people look for can have any one of these versions of XP, it is always recommended that you update to the latest Service Pack which is free. This will include security patches as well as other functionality updates that will address compatibility with newer technologies and other hardware related aspects too.
While XP has been around for a long time now, many computer users still prefer it over the newer Microsoft operating systems. The main support may have ended yet the extended support for XP will be around until April of 2014. If you keep your used laptop protected with antivirus software, a firewall and antispyware program then you will definitely be addressing the most prolific potential issues with any computer which connects to the Internet. There are used laptops under $100 which can run Vista and even Windows 7 so these options make it easy to save big and still utilize the OS of your choice. As with any product, there is some form of a relevant life cycle and the same goes for laptops and the operating systems which they use.