Allocation of file blk0001.dat is very inefficient. It produces more than 100 file fragments per minute on Windows XP 32 bit SP3 (NTFS partition). System becomes very slow and sometimes unresponsive. Hard disk activity is very high.
I encountered this problem in early versions of the client and it still remains in 0.5.2.
Here’s screenshot of the fragmentation after 5 minutes of block downloading - see file blk0001.dat. Note that I defragmented this file to one fragment before I ran the client. http://i.imgur.com/fMhWR.png