Support for XP ends next year, and what this means is that there will be no more updates available, INCLUDING security updates.
What this means is that should vulnerabilities be found in the OS (By Hackers for example) they will not be plugged by Microsoft, however providing you have good security software installed then this should not be too much of a problem in the near future. (Unless you regularly frequent dodgy sites)

With Windows 8 Microsoft is moving into providing the same time of update method as Apple provide for their computers. (A BETA version of the first update is already available from Microsoft (Free) with the completed version of it being scheduled for the end of August)

XP is over a decade old so it�s hardly surprising that Microsoft is dropping support for it, (Most other manufactures would have killed off support for it years ago) therefore moving on to a new OS is now a good time. (I would suggest Windows 7 Pro as Windows 8 is still pretty new and quite different OOTB then other Windows OS, plus Win 7 Pro has Virtual XP available for it which means in the unlikely event of something not working after upgrade, you can load in this software that will enable 99% of XP software to work OK on Win 7, however in most cases it will not be needed)

As to the adverts displaying then that has always been a problem, however do a search for Ad-Block software and it will solve most of the problem for you. (I suggest www.download.com as a good source)

Hope this helps

Bill