Posted by sooBlown
Found this post from google ;)
1. Start L4D, go to Options, and go to Steam Cloud.
2. Disable Steam Cloud if it is enabled.
3. Open C:\Program Files\Steam\steamapps\common\left 4 dead\left4dead\cfg
4. Open Config.CFG with Notepad
5. Modify crosshair "0" to crosshair "1" and save.
6. Enjoy your crosshairs again.
Also delete a file called autoexec.cfg located in Left 4 Dead/cfgs/
you could probably also just type crosshair "1" into console
The reason a reinstall didn't work is because the uninstall process usually keeps custom content.