We all know by now that WindowsXP has reached its "end-of-life" and will no longer be supported with updates. It's been around for a long time and if you're still using XP machines it really is a good idea to get something new (that hardware is getting pretty old too!). Of course there are reasons why that might not be possible. So, until you are able to upgrade here are a few tips to help ease the fear of a complete hacker invasion on your network between April 9 (the day after support ends) and the time you are able to upgrade. (highlights taken from this article - read the full text for more information)
As with any computer, an antivirus program is essential, and continue to update regularly.
The operating system is important but so is all the other software installed. Make sure to keep those installations updated as well.
Now is the time to let go of Internet Explorer - the version that works with XP is too old, leaving the door open to more attacks. Move on to Firefox or Chrome (or both - you don't have to limit yourself to just one!). The tip (in the article linked above) about disabling Java in browsers is also a good one.
Here's the best idea: if not done already, set up a "limited" user account on every XP machine, and use that instead of the administrator account. The limited user can't install, remove or make other system changes. If you've got "admin" and "public" accounts, be sure the "public" account isn't set up as an administrator (find the type of account by checking User Accounts in the Control Panel - if it is, change it). When changes are absolutely necessary log on as the admin, do what needs to be done, then log out and continue with the limited account.
As mentioned in the previous post, XP machines can still be useful for applications that don't involve the network; just be careful if you decide to keep them online.