I have to split an srt file (after manually splitting an oversized avi file). I've never done this before and I'd like to preserve the srt format, do you know any tool that would allow me to do it quickly and easily?

Tried Subtitle Workshop ?

Haven't used this feature myself but Load the srt and click Tools / Split Subtitle.

Just find the count at where the last line is said, do a search in wordpad for that line.

Rename the srt to txt.

Open with notepad.

Copy paste the first part to notepad, save.
Copy paste the second part the notepad.

Rename to the filename of the avi's with an srt extension.

Shouldn't be to off.