Thousands of duplicates

I've been trying to upload a database. The first 6 times I tried, it "appeared" that everything worked fine, but after the data was uploaded, it kept indicating that 0 entries were added. On the 7th try, it uploaded the db BUT, it duplicated each account dozens of times, so now I have more than 100,000 accounts listed.

So I guess I have 2 problems...

1) Why wouldn't my db upload properly? I followed the instructions but it kept telling me that 0 entries were added.

2) How do I delete all of these accounts (the 100,000+ that are now there)?

As you can imagine, I'm incredibly frustrated. I've been using salesforce.com for 2 years and never had a problem like this. Maybe switching was a really bad idea???