If it is in the WinPE phase, then this can happen. The boot image has a RAM requirement. For Vista RTM/WinPE2 x86, it was generally 512MB and x64 was 1GB. There are some situations where the amount of available RAM may be less than what is physically in the system. Some may be allocated to onboard hardware such as mass storage or video controller. Other RAM issues I've seen with Vista's PE is where the RAM is faulty or failing, or in some rare cases the timings are wrong (relabelled memory) or (on certain hardware) the timings are not matched. If this was during the HDD boot phase (2nd stage) then RAM wouldn't be so much of an issue.