I have a mac and want to get a windows pc could i be able to put all my data onto an external HDD and then upload it to the windows pc or am i gunna have to convert it or sumthing?

What format is the "data" in?

Standard file formats don't need any conversion, more exotic stuff and things like text files and program settings probably need some conversion.

Yes, you can use a FAT32-formatted external hard drive to transfer data between the computers.

Applications will not work, but .DOC, XLS, PPT and the like should work fine.