Compiler gives hundreds of "invalid character" suddenly

The following comes from the .log file you reported:

("C:\Documents and Settings\ander.biguri\Escritorio\Memoria\LaTeX\Memoria.aux"
! Text line contains an invalid character.
l.2 ! Text line contains an invalid character.

This means that the invalid character is found in the .aux file. Remove the file and retry.

Errors during a compilation may, sometimes, cause a corruption of the .aux file that is rewritten each time. The removal is a safe operation, which has the only consequence that some more LaTeX runs are needed for synchronizing cross references.

The following line of the error message (which you haven't shown) should show the character for example

! Text line contains an invalid character.
l.7 \begin{document}^^?

In fact I think in the default setup that is the only invalid character (delete, U+007F). So You need to delete the delete character.

Possibly your file has been saved in an incorrect encoding?