You could try putting all your data files (Word, Excel etc) (drag and drop them) on the D Drive, though probably this won't save enough space.
If you right click on My Computer and choose the Clean Up Disk (or something like it) option, this may help slim down the C drive too.
Have you loads of junk programs? Look in Control Panel and the Add/Remove Software option to clean up anything you don't need or haven't used.
You can get disk management software (often on a freebee CD with a computer magazine that may enable you to merge or re-size the partitions....but do a back-up first.
If all else fails, you could save your data, re-format the disk, and re-install Windows and Office, etc. Check to see if your Samsung has 'Samsung Recovery Solution' in the programmes folder: this may simplify this process. And finally, if considering the last option, you migh think of upgrading the hard disk to a bigger one: they're pretty cheap, and all you generally need to do is unscrew a cover on back of laptop, plug out the old HD and plug in the new, replace cover.
I assume you've also got updated anti-virus etc software: bugs can also slow things down.
Hope this helps.