The second Tuesday of this month brings something else in addition to Microsoft Security Updates. July 11th will also mark the end of Windows 98/ME technical support. It's now the end of their lifecycle.
According to IDC figures, there are an estimated 70 million 98/ME machines still in operation worldwide.
So what to do if you're still running an older OS? Make sure that you have a good antivirus product and a firewall. Check out Microsoft's Security At Home 98 & ME for more information.
If you're still holding on to older hardware because you're a hobbist, you might also consider trying out a lightweight distribution of Linux. Live CDs make it quite easy to give it a try.