I edited the above so it works
with version 3.70.
DELETE FROM ShoppingCartItem
DELETE FROM OrderNote
DELETE FROM RecurringPayment
DELETE FROM RewardPointsHistory
DELETE FROM [Order]
DELETE FROM PredefinedProductAttributeValue
DELETE FROM RelatedProduct
DELETE FROM ProductAttribute
DELETE FROM Picture
DELETE FROM Product_Category_Mapping
DELETE FROM Product
DELETE FROM PredefinedProductAttributeValue
DELETE FROM Product_Picture_Mapping
DELETE FROM Product_ProductAttribute_Mapping
DELETE FROM PredefinedProductAttributeValue
DELETE FROM Product_ProductTag_Mapping
DELETE FROM Product_SpecificationAttribute_Mapping
DELETE FROM ProductReview
DELETE FROM ProductReviewHelpfulness
Delete UrlRecord Where EntityName = 'Product'
DBCC CHECKIDENT (ShoppingCartItem, RESEED, 0)
DBCC CHECKIDENT (OrderNote, RESEED, 0)
DBCC CHECKIDENT (RecurringPayment, RESEED, 0)
DBCC CHECKIDENT (RewardPointsHistory, RESEED, 0)
DBCC CHECKIDENT ([Order], RESEED, 0)
DBCC CHECKIDENT (RelatedProduct, RESEED, 0)
DBCC CHECKIDENT (ProductAttribute, RESEED, 0)
DBCC CHECKIDENT (Picture, RESEED, 0)
DBCC CHECKIDENT (Product_Category_Mapping, RESEED, 0)
DBCC CHECKIDENT (Product, RESEED, 0)
DBCC CHECKIDENT (PredefinedProductAttributeValue, RESEED, 0)
DBCC CHECKIDENT (Product_Picture_Mapping, RESEED, 0)
DBCC CHECKIDENT (Product_ProductAttribute_Mapping, RESEED, 0)
DBCC CHECKIDENT (Product_SpecificationAttribute_Mapping, RESEED, 0)
DBCC CHECKIDENT (PredefinedProductAttributeValue, RESEED, 0)
DBCC CHECKIDENT (ProductReview, RESEED, 0)
DBCC CHECKIDENT (ProductReviewHelpfulness, RESEED,0)
Now if you also want to start clean and delete all Subcategories along with master category then
DELETE FROM Category;
DBCC CHECKIDENT (Category, RESEED, 0)
If you can't run the script because your hiding under the desk then use
http://sqlbackupandftp.com/sqltoexcel/Sql to Excel it's free.
You can take out your SQL products table directly, and convert it to straight to MS excel (can't escape excel).Then you can remove certain duplicates if you messed up your SKUs then upload back to SQL using use SQL Server 20XX Import and Export Data (64-bit). Or you can use the admin panel of Nop. Either way
Have fun.