Disclosure: I'm a certified accountant. It is not true that buying a computer is tax deductible. A computer is normally a capital expense. It is purchased and then depreciated over the useful life of the asset to emulate "using up" the asset over time. While this does reduce profits to the corporation and thus normally reduces their tax bill, saying that a computer is tax deductible is not true for businesses of any size under normal circumstances.
Well, if we're picking nits, any depreciating asset IS tax deductible - over time.
While the useful life of the asset is generally used for internal accounting purposes, for tax purposes it's not always relevant. (For those following along at home, businesses track depreciation on their own books using generally accepted accounting principles, but deduct depreciation expenses on their tax returns based on current tax codes.)
In this decade alone, the Canada Revenue Agency has placed computer hardware in four different asset classes with 30%, 45%, 55%, and 100% depreciation rates. Right now, computer purchases fall into a special class (class 52) which even waives the usual half year rule.
So, a computer is a 100% tax writeoff in year one, but still gets treated as a capital asset. I'd call that tax deductible. :)