I find it difficult to believe that the company would allow PCs to leave its offices with HDDs in a "current use condition". Many companies trash the HDDs rather than risk an inefficient or partial format.
Data Protection Act has no relevant interest in PC's with wiped HDDs.
Microsoft may have an interest in the use of its W'98 License, which is I believe called EULA, but as your company will have a multiple user license, I think it unlikely that they will pursue you.
Whether the data held held by yourselves is legally available to a purchasing company will depend on how you collected that data and for what use the purchasing company plans to use it.
There are a number of companies whose database is worth more than their product. Corporate Lawyers have made a fortune or two, whilst defining the availability of one companies data to another company.