what is the easiest way to view photos on the computer ?
If you're only using JPEG images, you can use the USB connection to transfer the images as usual (camera in playback mode, CHDK's USB remote setting disabled), otherwise you may use a card reader to copy your media files to your hard disc.
Allmost all viewer apps can handle JPEG images, e.g. the free XNView viewer; when using CHDK RAW files i suggest to enable CHDK's DNG support, the DNG image format is supported widely...
Do I have to swap al the times between the active partitions?
Only when access to "the other" partition is needed, e.g. when updating the CHDK files.