Try stopping all other programmes that are running such as antivirus. A good programme for this is EndItAll. Do a search from Google etc to find it. It is a free download.
Make sure you have plenty of spare disk space. DV will take around 1Gb of disk space for every 4 minutes captured. Also defragment the disk.
In an ideal world you would also be capturing to a dedicated disk with no programmes on it.
Make sure the DMA is enabled for your disk.
Try the above and let us know if this does not help.
Mark.