:O
Maybe your PC's crap. Your best bet is to ask, you know,
Steam. I mean, they have a support page and a forum right there.
If the game freezes at the title screen's loading thing, it's probably your graphic card. Portal is pretty graphics intensive with the lighting and the the portal graphics engine. Otherwise, I took 3 whole seconds of my time to google "Portal loading issues" and found another possible problem:
https://support.steampowered.com/kb_...9354-RTIB-6437