Securing files/folders on a laptop?

Issue: I have a laptop with Windows ME loaded. There are numerous files and/or folders which I'd like to keep secure while still allowing them to be easily read from, edited, and appended. The information stored should be tightly encrypted, in that it contains credit card info, social security info, tax records, and more. However, I don't want to enter a password for every single access to the data. Also, there is only one PCMCIA slot in this laptop, and it is already occupied (so we're looing for a software solution only).

Solutions?