Office 2003 SP3 will block multiple file formats from Microsoft to Corel. Microsoft has said this is by design as these older file formats are not safe. For the most part these are out of date formats rarely used however you may find you need access to these formats so to do that you must edit the registry to allow them.

File Formats blocked by Office SP3:

LotusandQuattroFiles	.wk1, .wk4, .wj3, wk1 FMT, .wks, .wk3, wk3 FM3, .wj2, .wq1, .fm3, .wj1
DifandSylkFiles	.dif, .slk
Word 97 and before
Excel 97 and before
PowerPoint 97 and before
Corel Draw	.cdr

If you need to enable these older formats in part or in whole all you need to do is edit the registry enabling the file formats. Instructions for editing the registry can be found here:

Enable File Formats Blocked By Office SP3