It is classed as an "Important" update which usually means a security one. It does, like all .net updates, take ages to download and install particularly on an XP machine with less than 1GB of RAM. I would turn off Anti-Virus and install it manually when you don`t actually need to use the machine.
When this first came out a couple of weeks ago several of my customers phoned me to say their computer needed a service because it had been slow for days. Guess what the culprit was? Just told them to leave the computer alone for a couple of hours whilst it installed.