There is an application called Zolt. Run XP with this application and see how it works out for you.
What it does is simple, it runs a service and adds an icon in the tray for easy enabling/disabling. As long as it is disabled, your computer will work normally. However, when enabled no-one is allowed to write to (any of) your hard disk(s) anymore. In essence, it turns a hard disk into a CD-ROM. The latest version of it that I could still find, was back in 2002(!) and it is still in my archive, so if you really want it I could PM it to you. For some reason, attaching it to this post seems impossible.
Try this software and be amazed about the type of sites you can visit without any chance of infection. At the time I tried it, I just had my 5 Mbit connection and a fresh XP installation. In those days it was bad in the Netherlands, the first time you would go on the net and immediately let your ant-virus software update itself, you would already infected by a(t least one) virus before that update was finished.
But with this software enabled not one virus was able to infect my system. To me this software looks like it disables write access at the Windows kernel level. I do remember though that 3 to 6 months after I downloaded this, I wanted to see if there were updates. The website of the creator just stated that he received a note from Microsoft, to stop development and distribution of the software immediately. Even (freeware) download sites didn't have links anymore.
This software comes with costs, though. Depending on how much software is installed in your XP installation that requires write access, your screen will be filled with Windows message boxes mentioning that: 'Application <insert name here> couldn't write to <insert folder here>.' in the best case scenario to application failures in the worst.
So only consider using this if you have no other option left than to keep running XP after the expiration date and don't mind clicking away Windows messages during the time this software is activated.
Or if you have a VM or clunker with XP you don't mind sacrificing, you might want to try it...or find out what makes it tick...or whatever.
Best see this as a last resort. The only serious consideration you should have in your mind is how to be able to upgrade to either a newer version of Windows or Linux. Please, give XP it's hard-earned and well-deserved rest.