Have a look at the account that sql server agent is running as. Which sql
server agent? Depends on push or pull. For Push it'll be the agent at the
distributor, and for pull the agent at the subscriber. If you are using
proxies then it'll be different and you'll need to consider them instead.
Once you have the account, make sure it has the correct pvivileges on the
distribution working folder so it can read the snapshot files.
HTH,
Paul Ibison