Crowding out theoretically occurs when the government expands its borrowing more to finance increased expenditure or tax cuts in excess of revenue (i.e., is engaged in deficit spending) crowding out private sector investment by way of higher interest rates. It represents one of the major controversies in modern Macroeconomics.
However, this crowding-out effect is moderated by the fact that government spending expands the market for private-sector products and thus stimulates – or "crowds in" – fixed investment (via the "accelerator effect"). This accelerator effect is most important when business suffers from unused industrial capacity, i.e., during a serious recession or a depression.
Crowding out can, in principle, be avoided if the deficit is financed by simply printing money, but this quickly leads to hyperinflation as seen in Germany in the period between WWI and WWII, or in Brazil before the introduction of the real.
http://en.wikipedia.org/wiki/Crowding_out_(economics)