Aiko - have you got lots of images in the presentation?
If so, it would be worth seeing if you can compress them as this can save huge amounts of space, esp if you're using digital camera pics or high resolution images.
1. Select a picure
2. If you're in Ppt '07 or 2010, click the Picture Tools tab and select Compress Pictures.
3. Choose your ideal resolution (it gives some hints in the dialog box)
4. Decide whether you want to delete the cropped areas of any pictures (PowerPoint hangs onto the information in case you want to un-crop later)
5. Click OK, save the file again and fingers crossed it's now much smaller!
This info is based on the article here that shows the steps and has screengrabs to help:
http://www.wiseowl.co.uk/blog/s142/compress_picture_images_microsoft_powerpoint_2007.htm
Good luck!