One usually gets this error because the parameters for the models in his cstrike/models folder doesn't match the server's. This can be caused by the slightest of discrepencies. I have seen this error occur from custom model installations (custom weapon models, etc.) and from corrupt map downloads (entering a CS_ITALY server that causes you to download a different version of the map--this creates a conflict with most servers because Counter-Strike uses downloaded models/maps before it uses the compressed versions in the GCF files).
I don't think I explained that very well. Anyway...
Find your Counter-strike directory (usually steamapps/user@email.com/Counter-strike/cstrike, or Counter-Strike Source/cstrike for Source) and delete the Models folder. You might have to re-download models for custom maps, but that won't take long and this should solve your problem.