when you install programmes,they all want to be on the c drive-change this to d when installing them(it will tell you it is being installed at,for instance,c:/programme files -click on the browse button,delete c ,replace with d.)
If you have XP Pro then you can convert your drive to a dynamic disk and resize the 'volume' through the Disk Manager. If you are using XP Home or XP Pro on a portable computer then this option is not available. Whats wrong with using the 90gb on your D drive
The only way to do this in XP is to Remove your D Drive click here then create say 3 new drives with the space (where the D Drive was)using Disk Managment click here Right click on the unallocated space and create a disk. Now mount one of the disks onto a folder on C Drive click here
PartitionMagic 8.0 is the best thing I have bought for effortless resizing and merging of partitions, anything less than 8.0 wont be able to cope with NTFS. I have had no trouble using this with Windows XP Home Edition.