If you haven't figured out from this blog, I'm a Windows Guy. But I have a dirty little secret: our corporate Internet presence is a Linux server. Recently I was talking to a Windows IIS developer at a medium-sized company and he was amazed that I would entrust our public face to such an "anarchy of developers." I asked him if he had ever administered a LAMP (Linux/Apache/MySQL/PHP) server and he admitted that he hadn't. I said to him that there were two key reasons that I use Linux: stability and maintainability.
Our corporate Linux server (Redhat Enterprise 3) has been rebooted once in the last five years and that was only because the power grid in the hosting facility was being upgraded. The current uptime count is 1,242 days, or almost three and a half years.
The pedants among you will point me to Netcraft's uptime list to show that Windows Servers can be kept up that long too. However, in my experience that's more than a rarity - it's almost a unique exception that requires Herculean efforts. In contrast, our corporate server was simply automatically updated as patches became available. All available patches have been installed (except kernel patches.) No special efforts were required to keep the Linux server running because of updates.
Much of this is possible because Linux does not have the concept of "in use" files. You'll never get an "Access denied" error because someone else has a file open. In practice, this means that shared libraries can be upgraded on the fly without affecting versions of those libraries that are in use by applications. Unix has allowed the replacement of open files for as long as I've used it, over 25 years.
So as you are sitting there thinking yourself smugly superior for your IIS server, take it from another Windows Guy: when your system absolutely, unquestionably has to stay running, Windows should probably not be your first choice.
You'll notice that the NetCraft list I cited earlier includes no LAMP servers. However, until fairly recently, most variants of Unix/Linux could not report more than 497 days of uptime. Thus the presence on the list of FreeBSD but not other variants. See the NetCraft uptime FAQ.
Postscript: Up until March 2007, the Uptime Project tracked who could keep a computer running the longest without rebooting. The winner was an OpenVMS system that had been running for nearly 12 years. Irish Rail allegedly had an OpenVMS machine up for 18 years. OpenVMS is also used by both the US Postal Service and by Amazon.com. Probably completely uninteresting for most of you, but I used VMS for about five years and it has a warm place in my heart.