Try this people, if simply restarting doesn't work.
Shut down Steam. Go into your steamapps folder and move all the gcf files (that's right, all 4 gigs) somewhere else. Start Steam and tell it to preload HL2. You'll see 0% completed, and then it will spend some time creating the gcf files again. When the preload has downloaded something (it should say 0.1% complete at least), shut down Steam, move the old gcf files back into SteamApps folder. Start Steam again - it will continue the preload normally.
Worked for me, and I was in the situation of many others, with HL2 completely preloaded and stuck on 88% today.
Shut down Steam. Go into your steamapps folder and move all the gcf files (that's right, all 4 gigs) somewhere else. Start Steam and tell it to preload HL2. You'll see 0% completed, and then it will spend some time creating the gcf files again. When the preload has downloaded something (it should say 0.1% complete at least), shut down Steam, move the old gcf files back into SteamApps folder. Start Steam again - it will continue the preload normally.
Worked for me, and I was in the situation of many others, with HL2 completely preloaded and stuck on 88% today.