What is the risk of having 100k euro in accounts in Greek banks and Cypriot banks?
Is there a guarantee?
I assume that the state guarantee for deposits doesn't hold when a country defaults.
I am going to clone an answer from another question that I wrote ;) and refer you to an article in the Wall Street Journal that I read this morning, What's at Stake in the Greek Vote, summarizing the likely outcome of the situation if a Euro exit looks likely after the election:
... we will see a full-fledged bank run. Greek banks would collapse ... The market exchange-rate would likely be two or three drachmas to the euro, which would double or triple the Greek price of imported goods within a few days. Prices of assets, including real-estate assets, would crumble. Those who moved their deposits abroad would be able to buy these assets cheaply, leading to a significant, regressive redistribution of Greek wealth.
In short, you'd lose about two-thirds of your savings unless you were storing them somewhere safe from the conversion. The article also predicts difficulty importing goods (other nations will demand to be paid in euro, not drachma) leading to disruption of trade and various supply shortages.
The best scenario where Greece does not leave the euro: In this scenario there is probably no risk, because either the ECB will print more money, or other countries will help Greece in some way.
The average scenario where Greece leaves the euro: All Bank accounts will be frozen and slowly turned into NEW DRACHMA, and your poor money after the conversion will be worth 50K euro at best (but probably much less).
There is also the worst scenario: The bank defaults too, and you will lose everything. Italy has a fund to protect deposits up to 100K euro (I don't know if you have something similar in Greece). However, a similar fund in Greece would be guaranteed by Greek banks and the Greek government, so you might not get much back regardless.