Gardening and Horticulture - Flower trimming help

I know very little about taking care of plants. I have a very beautiful puprle flower bush. It was planted years ago in front of a front window of the house. This time of year, it blooms with many beautiful purple flowers. I don't know what kind it is, only that it does seem to be fairly popular here in New Hampshire as I have seen lots of them. The flowers look like purple balls about the size of a softball.
They are now half way up the window and I need to trim them to see out the window. They are in a full bllom now.
When is the best time to trim them? Do you trim them on an angle? How far down the stem can I trim them and still have them bloom again next year? I need to trim them down 1-2 feet.
Thanks.

http://www.flowersbulbs.com/images/hyd_field2.jpg
Photo Credit: Oregon Coastal Flowers

The purple hydrangea blooms on old wood on stems from the summer before. New wood is wood produced this season. New buds are produced after July for next season. If you prune in fall, winter, or spring, you won't have blooms next June/July because buds were removed. Prune hydrangea in July. A general rule of thumb for pruning flowering shrubs is to do so when they quit blooming.

Remove dead stems every year. After 5 years to revitalize the bush, remove 1/3 of stems down to ground each summer. To reduce the size of the shrub, do so in June or July in order not to interfere with blooming. The problem is that the hydrangea will almost immediately return to its original size. Thus, it's best that hydrangeas are planted where they do not need pruning.

Cut back to base of stems or healthy bud. Cut so stems are of varying lengths so plant will not look like it was lopped off. If removing 1/3 of the canes to revitalize, cut to ground or back to a branch near the ground. Make clean cuts on an angle.

http://blog.nj.com/hg_impact_garden/2007/11/medium_g%20prune.jpg
Photo Credit:
Roxanne Washington, Newhouse News Service,
NJ Star Ledger

Thank you twelvepole. Good info. Those flowers look like what I have. They are now starting to "go by" as we call it here(meaning they are "wilting".)

If flowers are starting to go bye-bye, then it's time to prune.

When I read this my first thought was rhododendron. The leaves would be evergreen as opposed to the hydrangea shedding it's leaves for the winter. Could this be what you have?
http://en.wikipedia.org/wiki/Rhododendron

Newt