The default method for connecting to Excel data changed from DDE to OLEDB in
Word XP. Try either:
a. check Word Tools|Options|General|"Confrm conversion at open", then go
through the connection process again, and select the Excel via DDE option
when it is offered or
b. use numeric formatting switches to format your data, e.g. instead of

{ MERGEFIELD mynumber }

try

{ MERGEFIELD mynumber \# "$,0.00" }

Notice that the formar option will typically use the formatting you used in
Excel, whereas the latter option imposes new formatting which might or might
not be the same as what you have in Excel.