Probably not as serious as is made out.
The main security concerns on all Windows versions are those of users being duped into installing rogue software such as fake patches, browser plugins or antivirus programs, and of browser plugins such as Flash or Acrobat which have security holes.
To mitigate the former, bar ordinary users from installing software by making them limited users, or by way of a software restriction policy.
http://sourceforge.net/projects/softwarepolicy/ may be of help here.
If remaining on XP you should strongly discourage the use of Internet Explorer, as that will no longer be patched. Install Firefox or an alternative, which is supported by its vendor.