This usually means your startup disk is nearly full.

You can do various things to try fix this.

1. Clear out some files you don't need. A free program called CCleaner can be pretty useful here on a PC.

2. If you won't need to undo anything you have done already with the image, you can clear out the edit history by edit > purge > all

3. If you have another partition or drive with more space than the current startup, you can change the order of the default scratchdisks by going edit > preferences > plug-ins & scratch disks. Put your drive with the most free space first.