Due to the high level of compression on .WMV files you may find doing any editing will cause a fair amount of PQ loss. If you have the origional tapes and can recapture I would suggest you do in .AVI format as this is uncompressed from the tape (assuming it is a miniDV camcorder). If they were origionally recorded in a compressed format then you just need to do the best you can.
Depending on your final media then you may find Windows Movie Maker will do all you want. It is a free part of XP & Vista, but the XP version wont output to DVD etc. I'm not sure if you can do slow motion with it, but you should find all you need to know about it
Here.
If MM does not do what you want then you will probably need to buy a program. The normal suspects are Sony Vegus Studio, Pinnacle Studio, Adobe Premiere Elements, Ulead Video Studio or DVD MovieFactory etc etc. A search of this section of the forum should bring up loads of info on these and other programs.
Mark.