Hi Avast332,
While undoubtedly security on XP systems connected to the internet will be at an increasingly lower level, there are things you can do to decrease the risk. Remember that most, although not all, security problems on computers relate to user errors so vigilence is the key.
Keep your general software up to date by installing update patches when they are released - this includes btowser plugins too but as you use Chrome, this is automated, I believe.
Keep your security software up to date (virus definitions etc.) - don't rely on Microsoft Security Essentials but discard it in favour of a more robust anti-virus program like Avast - run a firewall (having Malwarebytes (or similar) in addition, as you have, is a good idea.
Do not use IE for browsing (you use Chrome which is good as is Firefox).
Be careful which sites you visit and what links you click.
Ideally, use a live Linux cd/dvd to boot your system when you need to carry out security sensitive internet activity such as online purchasing and banking - even better would be to use the live cd/dvd for all internet activity and to disable the network in XP.
These steps will help - they are not 100% guaranteed but then neither is using Windows 8.1.