You have a breach of contract action against the company. In Pennsylvania, there is a 4 year time limit (statute of limitations) in which you may bring a lawsuit to collect this debt. The 4 year time limit begins at the date of default (or when you become aware of the default) and notified the debtor of the default. So in your case, if you knew that the invoice was in default and unpaid since 2001 and you did nothing about it, then you may be barred from attempting to collect the debt 8 years later, due to the statute of limitation. But every case is specific with unique circumstances so you should contact a local attorney to discuss your case in more detail.
Also, note that the statute of limitations is an affirmative defense, meaning, it is up to the defendant to defend the suit by asserting the statute of limitations defense. You can always bring a legitimate law suit, it is just a matter if you will be successful against legal defenses. But as I stated, you should speak with an attorney about your case.
