If three PC's are doing it after a fresh install and all three have 512Mb of RAM then that suggests that you need more RAM as it's not only the WU check but also the downloads.
Keep in mind that after a fresh install pretty much everything is set by default to run at startup and that includes any bundled programs that come with most manufacturers PC's.
Get more RAM. Do a fresh install. Uninstall or turn off startup items. Download and install the updates. After that there should be no issue until you start installing programs. Some are resource hungry.
I've never seen checking for and downloading updates to be an issue except for cases of not enough RAM, too many startup items and dialup.