Are you trying to open the .pdf files by browsing to them on the web, i.e. by clicking on a link to them in internet explorer?
If so that, combined with you saying you cannot view bits of web pages, which may be made up of things like flash animations could suggest that your web browser has lost track of it's "plug in" programmes.
A reinstall of Flash and acrobat might fix it, but before you do you might want to check the security settings in your browser in case some how these have been changed to block some content you want to see. (Tools>Internet Options then the Security Tab under I.E. 6.0)
I suggest first trying to access a .pdf that is saved on your hard disk. If that works then you do not need to reinstall acrobat, just fix it's connection with the browser.