Explorer.exe is the standard shell in Windows....the 'thing' that provides you that typical desktop interface....start bar, icons, time, etc.
People who use alternate shells [such as LiteSTEP] are [or should be] aware that a Windows OS will 'run' without it...as the Kernel is what is 'running' and that's separate.
Fortunately the Task Manager is intentionally separate from Explorer as well so it's a 'back door' to get Explorer.exe running again.
With the new-found stability of XP and 2000 compared with the 9x vintage, you may just have found that Explorer crashing is a novelty, but in 9x it was commonplace and one of the incentives to adopt alternatives...