as said it could be due to a update ( automatically done on xp) which microsoft sometimes get wrong
you may have upgraded to internet explorer 7 (possibly automatically) and this causes a few oroblems as some sites don't recognise it )
http://www.microsoft.com/windows/pro...e/default.mspx is microsoft windows homesite and this has technical support etc faq
http://www.pcnineoneone.com/ is a good plain speaking pc help site
http://www.newbie.org/ as the title says newbies
these may not help they may do
sometimes its just a script in the website that has changed slightly and is wrong but rectifies itself in a day or so hence only probs on a few sites ( especially if of same type ( eg forums such as this) as most common forums all run on phpbb2 servers and are interlinked so it may be a central server error or similar.