I listed some of the regular questions that we see from our customers from PST/OST are,
1) How to reclaim the space from PST?
2) How to compact my PST file?
3) How to compact PST/OST files to eliminate deleted item space in Outlook?
4) My PST is filling my hard disk?
To conserve space on your hard disk, you can delete unwanted items from the various .pst/.ost files, and then compact the files to make them smaller. If you use Outlook 2002, 2003, 2007 and 2010 then try the following steps specified in the support KB.
Try this out and reclaim more room in your PC!!