Is this your first try with LW, or have you had it running before without a problem?

Did you download Pro from the official site, or did you download it from another source (if you downloaded from a different source, it's possible that the version you have is corrupted in some way)?

Do you have any antispyware/security apps which might be stopping LW from running (AVG shouldn't be a problem, if that's the only security program that you're using)?