If your want to, you can reserve a hotel booking as part of your conference registration. The hotels booked for delegates at the EuSEC 2010 are situated in the city centre of Stockholm and close to the conference venue Kungliga Tekniska Högskolan (KTH). Read more about this option here.
If you want to book your hotel stay yourself, you might find the official Stockholm's visitors guide accomodation site useful.
If you do not want to spend too much money on your accomodation, living in a hostel might be an option for you. There are several good hostels in Stockholm, including the STF Hostel af Chapman & Skeppsholmen, the STF Hostel Fridhemsplan, and the STF Hostel Zinkensdamm.
Notification of cancellation must be made in writing to Congrex, eusec2010@congrex.com. Cancellations will be accepted until 10 April 2010 with a refund of the deposit less a cancellation fee of SEK 500. After this date no refunds will be given.