Several Vista SP2 users are currently discussing the same problem in KellyB56's thread Windows Vista Update Hangs at "Checking For Updates".
Most users find that the step-by-step instructions on page 1 of m#l's thread Updates not working, it has been searching for updates for hours will get Windows Update working again. There are six Windows kernel-mode driver (Win32K.sys) updates listed in STEP # 2 of those instructions that must be manually installed on your computer to speed up Windows Update if any of those updates are currently missing.
And "speed up" is a very relative term. This workaround can reduce the "Checking for updates..." hang from days down to hours, but Note 6 of those instructions warns users to be prepared to let Windows Update run several hours after the Win32K.sys speed up patches are installed.
32-bit Vista Home Premium SP2 * Firefox v50.1.0 * NIS v18.104.22.168 * MBAM Premium v2.2.1