It says "UNMOUNTABLE_BOOT_VOLUME", and under technical information,

*** STOP: 0x000000ED (0x84F23620, 0xc000009c, 0x00000000, 0x00000000)

For background information, it was running very slowly before I restarted
it; I left the hotel running AVG for eight hours on the off chance my
computer had caught something, only to stop it when I returned. Other than
that, my computer wasn't displaying any noticable problems; when I tried to
restart it, however, it gave me this error instead of loading windows.

It's a Compaq Presario 2100 with no hardware changes made; I thought for a
moment that Windows Updates may have downloaded Service Pack 3 for me,
causing this problem (a family member had a similar problem caused by SP3,
though it was probably a different error number), but I discarded that
hypothesis on the basis that it doesn't match any of the error codes that
others are reporting.

Are there any thoughts on what my problem might be, and what steps I can
take to fix my computer? Though my computer is old, it is very dear to me,
and I have several files that I just can't stand the thought of losing. I
won't be able to use any suggestions involving my windows CD for several
days, though; I'm on vacation, and (for obvious reasons) didn't think I'd be
needing it with me.