Will posting in blog comment boxes with my website URL increase PR/serps. For example if i posted my URL in this blogs comment box would it help my PR/serps? http://labs.live.com/photosynth/blogs/CommentView,guid,721172f9-f94f-4cee-93ec-88e7274cca53.aspx
It depends if the blog has nofollow enabled or disabled. If it's disabled, that means that search engine bots will count it as a backlink. My own blog has nofollow disabled, so it actually attracts my visitors/readers to comment since every one they make gives them a free backlink.
even if it is do follow,you need to use a name that is a keyword,like the "live football" guy ,or it doesn't do a thing for serps.
As stated before if the blog is using nofollow which a lot of them do then no. Also, remember if you are posting your whole url then when someone does a search on your site url the blog will come up in the results, possibly higher than your own site.
There are many sites with dofollow enabled. Check in google for terms like dofollow. You will get many sites. Posting in them would help.