exactly what these "Windows errors" were, but on the face of it advising you to reinstall the operating system one week after delivery does sound a mite drastic. You would look to Poweroid for support with an OEM version of WindowsXP, so if they advised reinstallation I assume they exhausted all other avenues with you first?
Poweroid were liable to you for the faulty hard drive, but of course you have (inadvertantly) voided that liability by getting a repair carried out by another party. Once again you don't say what the repair consisted of, but I assume that the hard drive was replaced?
With the benefit of hindsight you may realise that you should have rejected the computer in the first instance, and insisted that Poweroid rectify the fault, or replaced the machine, as was your right under current consumer legislation. You have now circumvented all that by having a third party carry out the repair, and you've provided Poweroid with a reason for washing their hands of the affair.............or have you?
Based on what you say I believe you received less than fair treatment from Poweroid's helpdesk, after all, you had confirmation that the hard drive was faulty, and yet they still refused to accept liability to put things right. My personal view is that the very least Poweroid could do is reimburse you for the replacement of what was a faulty component. Under the terms of the supply of goods and services to consumers legislation any fault that occurs in your computer's hardware components withing the first six months after purchase would be deemed to have existed at the point of purchase, unless the supplier can prove otherwise. If you have the faulty drive perhaps Poweroid might take it back as evidence and pay you for the cost of providing and installing the new one - why don't you approach them on this basis?