You can transfer assets (such as a PC) purchased prior to the incorporation of a limited company into its books. The valuation would have to be a reasonable estimate of its value. You have to make sure that the assets become the legal property of the company so if, for example, you intended transferring a motor vehicle you would have to notify the DVLC of the change of ownership.
Care has to be taken if an existing business is being transferred to the limited company as the Inland Revenue will expect a realistic valuation of Goodwill (if any) which might give rise to Capital Gains Tax liability on the previous business owners. The assets transferred can be offset by liabilities with the net figure being credited (assuming assets exceed liabilities) into a directors loan account or being treated as consideration for the issue of shares in the limited company.