Halp.

[Hutch]

Reformat the computer and try again.

[VoX]

Hmm, in which case I'd say go with Cal's suggestion, re-install steam and delete the entire folder to make sure.

[ez64]

Uninstall steams and dont copy your game folders across,

Restart in all non windows items and services disabled in msconfig,

Install steam (fresh download from filehippo dont reuse another installer)
make sure steam custom installs to a new location not the default ie just prog files/steam2 to make sure it isnt registry problems.

right click on the folder properties and take permission of the folder with Full rights will post a how to if you dont know how to do this its under security permissions and advanced.

then run steam from the exe no shortcuts and run as admin.

Should fix your problem but make sure you dont let anything startup services or exe's apart from windows stuff. make sure before you install the new steam that you disable any anti viri you have.

if its fixed startup a block of new programs in msconfig that you normally have running, if you suddenly find its broken again you may have found your problem.

[Chazlene]

Restart in all non windows items and services disabled in msconfig

Urmm what do you mean by this? (Sorry, you're gonna have to slow down a lot for me).

And yeah I'm not sure about all that folder security stuff you mentioned.

[ez64]

Sorry

go to msconfig via run

then click the services tab

then check hide all ms services and then press disable all, then go onto the startup items tab and just disable all for now.

you will want to turn this stuff back on later but for the purpose of a clean steam install you will want them off. Restart your pc when it asks then follow the steam install procedure with any anti virus programs you have disabled.