HELP!!! It won't start! Why?

[UnicornPie]

I uninstalled Portal December of last year, so I could install other games. Now I installed Portal again, to play it again and when I try to run the game it won't start. Help, what should I do??

[Drew P. Wiener]

Your antivirus program could be blocking it

[Orange Bitflip]

Първоначално публикувано от TheBaconatorZ:

I uninstalled Portal December of last year, so I could install other games. Now I installed Portal again, to play it again and when I try to run the game it won't start. Help, what should I do??

One Hundred Million and More Things That Could Go Wrong
(abridged edition):

Your appcache folder could need a good ol' delete,
your clientregistry.blob could as well,
Portal could use a validation (go to the library, right click a game, properties, local files, verify, wait),
Portal could use a hard validation (same, except go to program files, steam, steamapps, <your Steam ID>, portal, and delete hl2.exe before validating),
Portal could need ran as administrator,
Your video card and/or video driver could be screwed up,
You could be trying to run Portal on 128MB of RAM.

One Hundred Million and More Things That Could Go Wrong has been abridged for the modern reader. We hope you enjoyed this fine read.