It's not necessarily drugs though, I was reading about that Spanish doctor and it seems what he would do is take blood from the riders, separate the red blood cells from the plasma and then the week before the race would put the red blood cells back in the rider (does that sound like a 5 year old has written it?) which would give them a performance boost.
I believe this is what Vinokourov got caught on - the fact there were 2 types of red blood cells meant he'd been given extra (of someone elses) to boost his performance.
This started back in the 1970s in athletics. The theory is that when you take a load of blood out of the athlete, the body then replaces the lost red blood cells (which carry oxygen round the body). Then, just before the race, you put the old red blood cells back and the athlete can get more oxygen to his muscules than he could previously.
For many years this was undetectable. I think the fact that Vinokourov was caught with someone else's red blood cells means it was a last-minute panic measure after he dropped back after the first week - using your own blood needs a few weeks preparation.